JP4758062B2 - Semiconductor device - Google Patents

Description

  The present invention relates to a structure and a driving method of a semiconductor device having a transistor. The present invention particularly relates to a structure and a driving method of a display device including a thin film transistor (hereinafter referred to as TFT) manufactured over an insulator. The present invention also relates to an electronic device using the semiconductor device having such a structure and a driving method.

  In recent years, development of display devices using light-emitting elements such as electroluminescence (EL) elements has been activated. An EL element has high visibility because it emits light by itself, and is suitable for thinning because it does not require a backlight necessary for a liquid crystal display (LCD) or the like, and has almost no restriction on a viewing angle.

  In general, the value of current flowing through the EL element and the light emission luminance of the EL element are in a proportional relationship. For this reason, a pixel configuration different from that of an LCD that controls the luminance with a voltage value has been proposed (see Patent Document 1).

  In addition, a driving circuit for writing a video signal to such a pixel has been proposed (see Patent Document 2).

International Publication No. 01/06484 Pamphlet International Publication No. 02/39420 Pamphlet

  In Patent Document 1, a current proportional to a video signal is input to a pixel, and a current value to be passed through an EL element is determined. However, this method is complicated because the drive circuit around the pixel needs to have a large number of current sources as shown in Patent Document 2 above. Also, the input of current to the pixel tends to be incomplete in the halftone input close to black, and the display quality of the halftone is lowered. Further, since the saturation region of the TFT connected in series with the EL element is used to control the current value of the EL element, the power consumption increases and the heat generation is also large. In view of the above disadvantages, an object of the present invention is to provide a pixel configuration and a driving method suitable for an EL element, in which a driving circuit around a pixel is simple, high in halftone display quality, and low in power consumption.

  In the present specification, one of the TFTs is referred to as a first electrode, and the other is referred to as a second electrode because a source and a drain of the TFT can have the same structure. Further, in this specification, it is called ON when a voltage exceeding a threshold is applied between the gate and the source of the TFT and a current flows between the source and the drain. In addition, when the voltage below the threshold is applied between the gate and the source of the TFT and no current flows between the source and the drain, it is called OFF. Note that in this specification, a TFT is given as an example of an element constituting a semiconductor device; however, the present invention is not limited to this. For example, a MOS transistor, an organic transistor, a bipolar transistor, a molecular transistor, or the like may be used. A mechanical switch may be used.

  The switch element has a state where a current flows between two electrodes and a state where it does not flow. In this specification, the flowing state is referred to as ON, and the non-flowing state is referred to as OFF. The two electrodes are called a first electrode and a second electrode, respectively. An electrode that controls ON and OFF is called a control electrode. However, the control electrode is not necessarily shown. In the present specification, when a TFT is used as a switch element, ON and OFF of the switch element corresponds to ON and OFF of the TFT. Note that the switch element is not limited to the TFT as an example. For example, a MOS transistor, an organic transistor, a bipolar transistor, a molecular transistor, or the like may be used. A mechanical switch may be used.

  In this specification, an EL element is given as an example of a light-emitting element, but the present invention is not limited to this. In addition to the EL elements, all of the light emitting elements and display elements, such as light emitting diodes, in which the current and the luminance are in a proportional relationship are related to this.

  In this specification, the discharge terminal 106 is shown as being connected to the anode of the EL element, but the present invention is not limited to this. When the discharge terminal 106 is connected to the cathode of the EL element, the EL threshold application TFT becomes a Pch TFT, and the voltage setting of each terminal also changes. For example, the charge bias terminal 622 has the same potential as the anode of the EL element when the EL threshold voltage is held in the EL threshold capacitor 621, and is lower than the anode of the EL element when the capacitor 101 is charged.

  The present invention is a method of emitting light by flowing the charge once accumulated in the capacitor element to the EL element. The light emission luminance of the EL element is determined by making the amount of charge flowing from the capacitive element per unit time to the EL element proportional to the light emission luminance. The amount of charge accumulated in the capacitor element is proportional to the voltage applied to the capacitor element. Therefore, by using a capacitor, it is possible to convert the proportional relationship between the light emission luminance of the EL element and the current value into a proportional relationship between the light emission luminance and the voltage value. As a result, the video signal can be input to the pixel by the voltage.

  The input of the video signal with voltage is simpler than the input of the video signal with current, and the drive circuit around the pixel is simple and halftone input close to black is easy to input.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
It has a capacitor element, charging means for charging the capacitor element with charge, and discharge means for discharging charge from the capacitor element to the light emitting element.

In the driving method of the semiconductor device of the present invention, in order to drive the light emitting element included in the pixel in the semiconductor device,
Charge the capacitive element,
By discharging the charge of the charged capacitive element to the light emitting element,
The light emitting element emits light.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A capacitive element; charging means for charging the capacitive element with charge; discharging means for discharging charge from the capacitive element to the light emitting element; and charging the capacitive element with the charging means or the capacitive element with the discharging means. And a gradation means for controlling the presence or absence of discharge.

The light emitting luminance of the light emitting element is controlled by controlling whether or not the capacitor element is charged or the light emitting element is discharged.

The charge charge amount is controlled by controlling the charging voltage to the capacitor element, and the light emission luminance of the light emitting element is controlled.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A capacitive element; charging means for charging the capacitive element with charge; discharging means for discharging charge from the capacitive element to the light emitting element; and gradation means for controlling the amount of charge charged to the capacitive element by the charging means. It is characterized by having.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A capacitive element; charging means for charging the capacitive element with charge; discharging means for discharging charge from the capacitive element to the light emitting element; and charging the capacitive element with the charging means or the capacitive element with the discharging means. And a light emitting element threshold value correcting means for adding the threshold voltage of the light emitting element to the charging voltage of the capacitor element.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A capacitive element; a charging means for charging the capacitive element; a discharging means for discharging from the capacitive element to the light emitting element; a gradation means for controlling an amount of charge charged to the capacitive element by the charging means; And a light emitting element threshold value correcting means for adding the threshold voltage of the element to the charging voltage of the capacitor element.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
By adding the threshold voltage of the light emitting element to the charging voltage to the capacitor element, the influence of the threshold voltage of the light emitting element is corrected.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A capacitive element; charging means for charging the capacitive element with charge; discharging means for discharging charge from the capacitive element to the light emitting element; and charging the capacitive element with the charging means or the capacitive element with the discharging means. Gradation means for controlling the presence or absence of discharge from the light emitting element, light emitting element threshold value correcting means for adding the threshold voltage of the light emitting element to the charging voltage of the capacitor element, and transistor threshold value for correcting the threshold voltage of the transistor included in the charging means Voltage correction means.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A capacitive element; a charging means for charging the capacitive element; a discharging means for discharging from the capacitive element to the light emitting element; a gradation means for controlling an amount of charge charged to the capacitive element by the charging means; The light emitting element threshold value correcting means for adding the threshold voltage of the element to the charging voltage of the capacitor element, and the transistor threshold voltage correcting means for correcting the threshold voltage of the transistor included in the charging means.

The driving method of the semiconductor device of the present invention is as follows:
The capacitor is charged by a current flowing from the drain to the source of the transistor,
A driving method of a semiconductor device in which charging is stopped when a potential difference between a source electrode and a gate electrode of the transistor becomes smaller than a threshold voltage of the transistor,
The influence of the threshold voltage of the transistor is corrected by applying the threshold voltage of the transistor in advance to the gate voltage of the transistor.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A charge switch, a discharge switch, a capacity line, and a charge terminal;
A first electrode of the capacitive element is electrically connected to a first electrode of the charge switch and a second electrode of the discharge switch; a second electrode is connected to the capacitive line;
A second electrode of the charge switch is connected to the charge terminal;
A first electrode of the discharge switch is connected to the light emitting element;
The capacitor line and the charging terminal are different in potential.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A gradation switch, a writing switch, a signal line, a gradation capacitance, and a gradation capacitance line;
The control electrode of the gradation switch is electrically connected to the second electrode of the writing switch and the first electrode of the gradation capacitor, and the first electrode and the second electrode are connected to the capacitive element and the charge. Provided between the terminal, the capacitive element and the capacitive line, or between the capacitive element and the light emitting element;
A first electrode of the write switch is electrically connected to the signal line;
The gradation capacitor is electrically connected to the gradation capacitor line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A gray scale switch, a write switch, a signal line, a gray scale capacity, a gray scale capacity line, an erase switch, and an erase line;
The control electrode of the gradation switch is electrically connected to the second electrode of the write switch, the second electrode of the erase switch, and the first electrode of the gradation capacitor, and the first electrode and the second electrode The electrode is provided between the capacitive element and the charging terminal, the capacitive element and the capacitive line, or between the capacitive element and the light emitting element,
A first electrode of the write switch is electrically connected to the signal line;
A first electrode of the erase switch is electrically connected to the erase line;
The gradation capacitor is electrically connected to the gradation capacitor line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A gradation transistor, a writing switch, a signal line, a gradation capacitance, and a gradation capacitance line;
The gate of the gradation transistor is electrically connected to the second electrode of the writing switch and the first electrode of the gradation capacitor, and the first electrode and the second electrode are the capacitance element and the charging terminal. Between
A first electrode of the write switch is electrically connected to the signal line;
The gradation capacitor is electrically connected to the gradation capacitor line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
An EL threshold capacity, a charging bias terminal, an EL threshold application switch, an EL threshold application transistor, and an EL threshold take-in switch;
A gate of the EL threshold voltage application transistor is electrically connected to a first electrode of the EL threshold voltage application switch, and the first electrode and the second electrode are provided between the capacitive element and the charging terminal;
A second electrode of the EL threshold value application switch is electrically connected to a first electrode of the EL threshold capacity and a second electrode of the EL threshold value take-in switch;
A first electrode of the EL threshold value taking switch is electrically connected to the light emitting element;
A second electrode of the EL threshold capacitance is electrically connected to the charging bias terminal;
The charging bias terminal is characterized in that the potential is changed according to the presence or absence of electrical conduction of the EL threshold value taking-in switch.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A gradation transistor, a writing switch, a signal line, a gradation capacitance, a gradation capacitance line, an EL threshold capacitance, an EL threshold application switch, and an EL threshold take-in switch;
A gate of the gradation transistor is electrically connected to a first electrode of the writing switch, and the first electrode and the second electrode are provided between the capacitor element and the charging terminal;
A second electrode of the EL threshold value application switch is electrically connected to a first electrode of the EL threshold capacity and a second electrode of the EL threshold value take-in switch;
A first electrode of the EL threshold value taking switch is electrically connected to the light emitting element;
A second electrode of the EL threshold capacitance is electrically connected to the second electrode of the write switch and the first electrode of the grayscale capacitance;
A first electrode of the write switch is electrically connected to the signal line;
The gradation capacitor is electrically connected to the gradation capacitor line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A transistor threshold capacitance, a transistor threshold first capture switch, a transistor threshold second capture switch, a transistor threshold third capture switch, a transistor threshold fourth capture switch, and a transistor threshold capacitance line;
The transistor threshold capacitance is provided between a gate of the EL threshold application transistor and a first electrode of the EL threshold application switch;
A first electrode of the transistor threshold first take-in switch is electrically connected to a side of the transistor threshold capacitance opposite to a side to which a gate of the EL threshold application transistor is connected;
A second electrode of the transistor threshold first take-in switch is electrically connected to a first electrode of the EL threshold application transistor and a first electrode of the transistor threshold second take-in switch;
The transistor threshold third take-in switch is electrically connected between the second electrode and the gate of the EL threshold application transistor;
Before the transistor threshold fourth take-in switch, provided between the second electrode of the EL threshold application transistor and the charging terminal,
A second electrode of the transistor threshold second take-in switch is connected to a transistor threshold capacitance line;
The transistor threshold capacitance line has a fixed potential.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A transistor threshold capacitance, a transistor threshold first capture switch, a transistor threshold second capture switch, a transistor threshold third capture switch, a transistor threshold fourth capture switch, and a transistor threshold capacitance line;
The transistor threshold capacitance is provided between a gate of the gradation transistor and a first electrode of the EL threshold application switch;
A first electrode of the transistor threshold first take-in switch is electrically connected to a side of the transistor threshold capacitance opposite to a side to which a gate of the gradation transistor is connected;
A second electrode of the transistor threshold first take-in switch is electrically connected to a first electrode of the gradation transistor and a first electrode of the transistor threshold second take-in switch;
The transistor threshold third take-in switch is electrically connected between the second electrode and the gate of the gradation transistor;
Before the transistor threshold fourth take-in switch, provided between the second electrode of the EL threshold application transistor and the charging terminal,
A second electrode of the transistor threshold second take-in switch is connected to a transistor threshold capacitance line;
The transistor threshold capacitance line has a fixed potential.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
Having a capacity line and a charging terminal,
In a charged state, the first electrode of the capacitor element is connected to the charging terminal, the second electrode is connected to the capacitor line,
In a discharged state, the first electrode of the capacitor element is connected to the light emitting element, the second electrode is connected to the capacitor line,
The capacitor line and the charging terminal are different in potential.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A gradation switch, a signal line, a gradation capacitance, and a gradation capacitance line;
In the gradation writing state, the control electrode of the gradation switch is electrically connected to the first electrode of the gradation capacitance and the signal line,
The first electrode and the second electrode are provided between the capacitor element and the charging terminal, the capacitor element and the capacitor line, or between the capacitor element and the light emitting element,
The second electrode of the gradation capacitor is electrically connected to the gradation capacitor line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A gradation switch, a signal line, a gradation capacitance, a gradation capacitance line, and an erasing line;
In the gradation writing state, the control electrode of the gradation switch is electrically connected to the first electrode of the gradation capacitance and the signal line,
The first electrode and the second electrode are provided between the capacitor element and the charging terminal, the capacitor element and the capacitor line, or between the capacitor element and the light emitting element,
A second electrode of the gradation capacitor is electrically connected to the gradation capacitance line;
In the gradation erase state, the control electrode of the gradation switch is electrically connected to the first electrode of the gradation capacitor and the erase line,
The first electrode and the second electrode are provided between the capacitor element and the charging terminal, the capacitor element and the capacitor line, or between the capacitor element and the light emitting element,
The second electrode of the gradation capacitor is electrically connected to the gradation capacitor line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A gradation transistor, a signal line, a gradation capacitance, and a gradation capacitance line;
In the gradation writing state, the gate of the gradation transistor is electrically connected to the first electrode of the gradation capacitance and the signal line;
The first electrode and the second electrode are provided between the capacitive element and the charging terminal,
The second electrode of the gradation capacitor is electrically connected to the gradation capacitor line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
An EL threshold capacity, a charging bias terminal, and an EL threshold application transistor;
In the charged state, the gate of the EL threshold voltage application transistor is electrically connected to the first electrode of the EL threshold capacity, and the second electrode of the EL threshold capacity is maintained at the first potential,
In the discharge state, the first electrode having the EL threshold capacitance is electrically connected to the light emitting element, and the second electrode is held by the second electrode.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
A gradation transistor, a signal line, a gradation capacitance, a gradation capacitance line, and an EL threshold capacitance;
In the gradation writing state, the gate of the gradation transistor is electrically connected to the first electrode of the gradation capacitor and the signal line, and the first electrode and the second electrode are connected to the capacitor element and the charging terminal. The second electrode of the gradation capacitor is electrically connected to the gradation capacitor line, and the gradation capacitor line is kept at the first potential;
The gradation capacitor and the EL threshold capacitance are provided between the gate of the gradation transistor and the gradation capacitance line in the charged state, and the gradation capacitance line is maintained at a first potential.
In the discharge state, the first electrode having the EL threshold capacitance is connected to the light emitting element, and the second electrode is maintained at the first or second potential.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
Having a transistor threshold capacity;
The EL threshold capacitance and the transistor threshold capacitance are provided between the charging bias terminal and the gate of the EL threshold application transistor in the charged state,
In the TFT threshold value acquisition state, the gate and the first electrode of the EL threshold voltage application transistor are maintained at the third potential, the second electrode is maintained at the fourth potential, and the third potential is applied to the transistor threshold capacitance. And the fourth potential is charged.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
Having a transistor threshold capacity;
The gradation capacitance and the transistor threshold capacitance are provided between the charging bias terminal and the gradation transistor gate in the charged state,
In the TFT threshold value acquisition state, the gate and the first electrode of the gradation transistor are maintained at the first potential, the second electrode is maintained at the second potential, and the first potential is applied to the transistor threshold capacitance. A potential difference between the second potentials is charged.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
Having a transistor threshold capacity;
In the charged state, the gradation capacitance, the EL threshold capacitance, and the transistor threshold capacitance are provided between the charging bias terminal and the gate of the gradation transistor,
In the TFT threshold acquisition state, the gate and the first electrode of the gradation transistor are maintained at the third potential, the second electrode is maintained at the fourth potential, and the third potential is added to the transistor threshold capacitance. It is characterized in that the potential difference of the fourth potential is charged.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
The pixel includes a capacitor element, first and second switch elements, a capacitor line, and a charge supply line.
The first electrode of the first switch element is connected to a charge supply line, the second electrode is connected to the first electrode of the capacitor element and the first electrode of the second switch element,
A second electrode of the capacitive element is connected to the capacitive line;
The second electrode of the second switch element is connected to the light emitting element.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
The pixel includes first and second capacitor elements, first to fourth switch elements, a capacitor line, a charge supply line, and a signal line.
The first electrode of the first switch element is connected to a charge supply line, and the second electrode is connected to the first electrode of the first capacitor element and the first electrode of the second switch element. And
A second electrode of the first capacitor element is connected to the capacitor line;
A second electrode of the second switch element is connected to a first electrode of the third switch element;
A first electrode of the third switch element is connected to the light emitting element, and a control electrode is connected to the first electrode of the second capacitor element and the second electrode of the fourth switch element;
A second electrode of the second capacitor element is connected to the capacitor line;
The first electrode of the fourth switch element is connected to the signal line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
The pixel includes first and second capacitor elements, first to third switch elements, a TFT, a capacitor line, a charge supply line, and a signal line.
The second electrode of the TFT is connected to the charge supply line, the first electrode is connected to the first electrode of the first switch element, and the gate is connected to the second electrode of the third switch element. Connected to the first electrode of the second capacitive element;
The second electrode of the first switch element is connected to the first electrode of the first capacitor element and the first electrode of the second switch element;
A second electrode of the second switch element is connected to the light emitting element;
A first electrode of the third switch element is connected to the signal line;
The second electrodes of the first and second capacitor elements are connected to the capacitor line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
The pixel includes first to third capacitor elements, first to sixth switch elements, a TFT, a capacitor line, a charge supply line, a signal line, and a charge voltage line.
The second electrode of the TFT is connected to a charge supply line, the first electrode is connected to the first electrode of the first switch element, and the gate is connected to the first electrode of the fifth switch element. And
The second electrode of the first switch element is connected to the first electrode of the first capacitor element and the first electrode of the second switch element;
The second electrode of the second switch element is connected to the first electrode of the third switch element and the second electrode of the fourth switch element;
A second electrode of the third switch element is connected to the light emitting element, and a control electrode is connected to the first electrode of the third capacitor element and the second electrode of the sixth switch element;
The first electrode of the fourth switch element is connected to the first electrode of the second capacitor element and the second electrode of the fifth switch element;
A first electrode of the sixth switch element is connected to the signal line;
The second electrodes of the first and third capacitors are connected to the capacitor lines;
The second electrode of the second capacitor element is connected to the charging voltage line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
The pixel includes first to third capacitor elements, first to fifth switch elements, a TFT, a capacitor line, a charge supply line, and a signal line.
The second electrode of the TFT is connected to the charge supply line, the first electrode is connected to the first electrode of the first switch element, and the gate is connected to the first electrode of the fourth switch element. connection,
The second electrode of the first switch element is connected to the first electrode of the first capacitor element and the first electrode of the second switch element;
A second electrode of the second switch element is connected to the second electrode of the third switch element and the light emitting element;
The first electrode of the third switch element is connected to the first electrode of the second capacitor element and the second electrode of the fourth switch element;
The second electrode of the second capacitor element is connected to the first electrode of the third capacitor element and the second electrode of the fifth switch element;
A first electrode of the fifth switch element is connected to the signal line;
The second electrodes of the first and third capacitor elements are connected to the capacitor line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
The pixel includes first to fourth capacitor elements, first to ninth switch elements, a TFT, a capacitor line, a charge supply line, a signal line, and a charge voltage line.
The second electrode of the TFT is connected to a charge supply line and the first electrode of the eighth switch element, and the first electrode is connected to the first electrode of the first switch element and the seventh electrode. The second electrode of the switch element is connected to the first electrode of the ninth switch element, and the gate is the second electrode of the eighth switch element and the first electrode of the fourth capacitor element And connect to
The second electrode of the first switch element is connected to the first electrode of the first capacitor element and the first electrode of the second switch element;
The second electrode of the second switch element is connected to the first electrode of the third switch element and the second electrode of the fourth switch element;
A second electrode of the third switch element is connected to the light emitting element, and a control electrode is connected to the first electrode of the third capacitor element and the second electrode of the sixth switch element;
The first electrode of the fourth switch element is connected to the first electrode of the second capacitor element and the second electrode of the fifth switch element;
The first electrode of the fifth switch element is connected to the first electrode of the seventh switch element and the second electrode of the fourth capacitor element;
A first electrode of the sixth switch element is connected to the signal line;
The second electrode of the first and third capacitor elements and the second electrode of the ninth switch element are connected to the capacitor line,
The second electrode of the second capacitor element is connected to the charging voltage line.

A semiconductor device of the present invention is a semiconductor device having a pixel for driving a light emitting element,
The pixel includes first to fourth capacitor elements, first to eighth switch elements, a TFT, a capacitor line, a charge supply line, and a signal line.
The second electrode of the TFT is connected to the charge supply line and the first electrode of the seventh switch element, and the first electrode is connected to the first electrode of the first switch element and the sixth electrode. Connected to the second electrode of the switch element and the first electrode of the eighth switch element, and the gate is connected to the first electrode of the fourth capacitor element and the second electrode of the seventh switch element. Connected to the electrode,
The second electrode of the first switch element is connected to the first electrode of the first capacitor element and the first electrode of the second switch element;
A second electrode of the second switch element is connected to the second electrode of the third switch element and the light emitting element;
The first electrode of the third switch element is connected to the first electrode of the second capacitor element and the second electrode of the fourth switch element;
A first electrode of the fourth switch element is connected to a second electrode of the fourth capacitor element and a first electrode of the sixth switch element;
The second electrode of the second capacitor element is connected to the first electrode of the third capacitor element and the second electrode of the fifth switch element;
A first electrode of the fifth switch element is connected to the signal line;
The second electrode of the first and third capacitor elements and the second electrode of the eighth switch element are connected to the capacitor line.

  According to the present invention, a complicated current source is not required by using voltage-current conversion by a capacitive element as a current source for driving an EL element having a proportional relationship between current and luminance. In addition, the display quality of the halftone was increased by the signal input with voltage. Further, since the voltage-current conversion by the capacitive element is linear, the EL element is accurately controlled by the voltage without requiring special processing. Furthermore, low power consumption is realized by not using the saturation region of the TFT as a current source.

(Embodiment 1)
FIG. 1 shows an embodiment of the present invention. The object of the present embodiment is to repeatedly charge and discharge the capacitive element by using a charging unit and a discharging unit, and to supply a constant current to the EL element.

The pixel that drives the EL element is
The capacitor 101, the capacitor line 102, the charge switch 103, the charge terminal 104, the discharge switch 105, and the discharge terminal 106 are included.

  The second electrode of the capacitor 101 is connected to the capacitor line 102, and the first electrode is connected to the second electrode of the charge switch 103 and the first electrode of the discharge switch 105. A first electrode of the charging switch 103 is connected to the charging terminal 104. The second electrode of the discharge switch 105 is connected to the discharge terminal 106. The discharge terminal 106 is connected to an EL element (not shown). The capacitor line 102 is kept at a certain voltage, and the charging terminal 104 is kept at a voltage different from that of the capacitor line 102.

  In the present embodiment, the charging means indicates the charging switch 103, and the discharging means indicates the discharging switch 105.

  The operation will be described. In this embodiment, an operation of repeating two states is performed.

  The first state is intended to charge the capacitive element 101. The second state aims at discharging from the capacitive element 101 to the EL element.

In the first state, the charge switch 103 is turned on and the discharge switch 105 is turned off. At this time, a potential difference between the charging terminal 104 and the capacitor line 102 is applied to both ends of the capacitor 101. When the capacitance of the capacitive element 101 is C, and the potential difference between the charging terminal 104 and the capacitive line 102 is V1, the charge amount Q1 charged by the capacitive element 101 is
Q1 = CV1
It becomes.

  In the second state, the charge switch 103 is turned off and the discharge switch 105 is turned on. At this time, the charge stored in the capacitor 101 flows from the discharge terminal 106 to the EL element.

If the potential difference between both ends of the capacitive element 101 after discharge is V2 (V1> V2), the charge amount Q2 remaining in the capacitive element 101 after discharge is
Q2 = CV2
In the end, the amount of charge Q discharged and flowing to the EL element is
Q = Q1-Q2 = C (V1-V2)
It becomes.

When the above operation is repeated at the frequency f, the amount of current I flowing through the EL element is
I = fQ = fC (V1-V2)
It becomes. From this, the amount of current flowing through the EL element is as follows: the potential difference between the charging terminal 104 and the capacitor line 102 is V1, the potential difference V2 across the capacitor element 101 after discharge, the frequency f, and the capacitance C of the capacitor element 101. To decide. In order to obtain a certain light emission luminance, a current value I that must be passed through the EL element to emit light at that luminance is obtained, and potential differences V1 and V2 and a frequency f are determined therefrom. However, since the potential difference V2 is automatically determined by the characteristics of the EL element and the frequency f, and the capacitance C of the capacitor 101 is also determined by the element characteristics, the potential difference V1 and the frequency f are actually determined.

  By the way, an EL element is an element in which current and luminance are proportional. Further, the EL element has non-uniformity in voltage / current characteristics due to deterioration or the like. Therefore, in order to cause the EL element to emit light with a constant luminance, driving using a current source is common. In Patent Document 2, a current source is disposed in an external drive circuit or an internal drive circuit of a display device, and a current is supplied to a pixel that drives an EL element. However, this method causes problems such as high cost, large display device, and easy occurrence of defects because the drive circuit is complicated.

  Further, the driving circuit and the pixel are at a distance of about several centimeters, and the parasitic capacitance is large. For this reason, when a minute current is supplied from the driver circuit to the pixel, a normal current does not reach the pixel in a short time, and halftone display close to black cannot be performed, resulting in a deterioration in display quality.

  As described above, there are many drawbacks in the method of flowing current from the current source outside the pixel to the pixel.

  In the present embodiment, a current source is realized in a pixel by performing voltage-current conversion using the capacitive element 101. Since it can be driven by voltage from outside the pixel, the driving circuit outside the pixel is simple, and there is no problem as in a driving circuit having a current source.

  Further, when a voltage is applied to the pixel from the driver circuit, the signal reaches the pixel with a delay time according to the time constant of the wiring. Since the delay time is not affected by the voltage value, even if the voltage is very small, the signal reaches the pixel in a short time as in the case of the high voltage, so that halftone display close to black is possible and display quality does not deteriorate.

  Also, when voltage-current conversion is performed using a TFT, it is difficult to control the current by voltage because the voltage-current characteristic of the TFT is non-linear. Furthermore, since it is necessary to operate the TFT in a saturation region, power consumption increases and heat generation is also large. On the other hand, in this embodiment, voltage-current conversion is realized by using a capacitive element. Since the voltage-current characteristic of the capacitive element is linear, it is easy to control the current by voltage.

  In general, in order to control the current value of an EL element, a TFT connected in series with the EL element is driven in a saturation region to supply current. Since the saturation region of the TFT is used, the power supply voltage is increased and the power consumption is increased. Also, the heat generation is large. In the present embodiment, a capacitive element is used to control the current value, so that power consumption can be kept low and heat generation can be reduced as compared with the case where a TFT is used.

In the present embodiment, a configuration different from the configuration shown in the present embodiment may be used. In the first state, the first electrode of the capacitor 101 is electrically connected to the charging terminal 104, the second electrode is electrically connected to the capacitor line 102, and the charging terminal 104 and the capacitor line 102 are fixed differently. Any potential may be used. In the second state, the first electrode of the capacitor 101 is electrically connected to the EL element, the second electrode is electrically connected to the capacitor line 102, and the capacitor line 102 only needs to have a fixed potential.
(Embodiment 2)
FIG. 1, FIG. 2 (A), and FIG. 2 (B) show another embodiment of the present invention. In the present embodiment, charging and discharging of the capacitor element are repeated by using the charging unit and the discharging unit, a constant current is applied to the EL element, and the time during which the current is supplied according to the light emission gradation is changed by the gradation unit. It aims at obtaining the brightness | luminance of.

  In this embodiment, the charging means and discharging means are the same as those shown in FIG. Further, since the configuration has already been shown, it will be omitted.

  FIG. 2A shows a gradation means for controlling the light emission gradation of the EL element.

  A gradation switch 207, a writing switch 208, a signal line 209, a gradation capacitor 210, and a gradation capacitor line 211 are provided. P and P ′ are terminals to be inserted at positions a to f shown in FIG.

  The first and second electrodes of the gradation switch 207 are provided at any one of a to f in FIG. 1, and the control electrode is the second electrode of the writing switch 208 and the first electrode of the gradation capacitor 210. It is connected to the electrode. A first electrode of the writing switch 208 is connected to the signal line 209. The second electrode of the gradation capacitor 210 is connected to the gradation capacitor line 211.

  The voltage of the gradation capacitor line 211 is not particularly limited as long as it is a fixed potential.

  A voltage for turning on or off the gradation switch 207 is applied to the signal line 209. The gradation is expressed by the time ratio of ON / OFF of the gradation switch.

  FIG. 2B shows another gradation means for controlling the light emission gradation of the EL element.

  A gradation switch 207, a writing switch 208, a signal line 209, a gradation capacitor 210, a gradation capacitor line 211, an erase switch 212, and an erase line 213 are provided. P and P ′ are terminals to be inserted at positions a to f shown in FIG.

  The first and second electrodes of the gradation switch 207 are provided at any one of a to f in FIG. 1, and the control electrode is the second electrode of the writing switch 208 and the first electrode of the gradation capacitor 210. The electrode is connected to the second electrode of the erase switch 212. A first electrode of the writing switch 208 is connected to the signal line 209. The second electrode of the gradation capacitor 210 is connected to the gradation capacitor line 211. The first electrode of the erase switch 212 is connected to the erase line 213.

  The voltage of the gradation capacitor line 211 is not particularly limited as long as it is a fixed potential. The voltage of the erase line 213 is not particularly limited as long as it is a voltage that turns off the gradation switch 207.

  A voltage for turning on or off the gradation switch 207 is applied to the signal line 209. The gradation is expressed by the time ratio of ON / OFF of the gradation switch.

  One of the gradation means shown in FIGS. 2A and 2B is inserted at the positions indicated by a to f in FIG.

  FIG. 3 shows an example in which gradation means is inserted. In the example, the gradation means of FIG. 2A is inserted at the position indicated by e in FIG. The same applies to the case of insertion at positions a to d and f.

  The operation will be described. First, a case where FIG. 2A is used as the gradation means is shown. When FIG. 2A is used, an operation of repeating three states is performed.

  The first state is intended to write light emission and non-light emission signals into the gradation capacitor 210 as a signal for controlling the gradation of the EL element. The second state is intended to charge the capacitive element 101. The third state aims at discharging from the capacitive element 101 to the EL element.

  In the first state, the writing switch 208 is turned on, and a voltage for turning on or off the gradation switch 207 is applied to the signal line 209 according to the gradation. When the writing switch 208 is turned OFF, the voltage of the signal line 209 is held in the gradation capacitor 210.

In the second state, the charge switch 103 is turned on and the discharge switch 105 is turned off. At this time, if the insertion position of the first and second electrodes PP ′ of the gradation switch 207 is either a or b or c to f in FIG. 1 and the gradation switch 207 is ON, the capacitor 101 A potential difference between the charging terminal 104 and the capacitor line 102 is applied to both ends. When the capacitance of the capacitive element 101 is C, and the potential difference between the charging terminal 104 and the capacitive line 102 is V1, the charge amount Q1 charged by the capacitive element 101 is
Q1 = CV1
It becomes. In addition, if the insertion positions of the first and second electrodes PP ′ of the gradation switch 207 are cf in FIG. 1 and the gradation switch 207 is OFF, there is no charge charging.

  In the third state, the charge switch 103 is turned off and the discharge switch 105 is turned on. At this time, the insertion position of the first and second electrodes PP ′ of the gradation switch 207 is either a to c or f in FIG. 1 and the gradation switch 207 is ON, or e or d. If there is, the charge stored in the capacitor 101 flows from the discharge terminal 106 to the EL element. Further, if the insertion position of the first and second electrodes PP ′ of the gradation switch 207 is any of ac to f or f in FIG. 1 and the gradation switch 207 is OFF, there is no charge discharge.

When the gradation switch 207 is ON, assuming that the potential difference between both ends of the capacitive element 101 after discharge is V2 (V1> V2), the charge amount Q2 remaining in the capacitive element 101 after discharge is
Q2 = CV2
In the end, the amount of charge Q discharged and flowing to the EL element is
Q = Q1-Q2 = C (V1-V2)
It becomes. When the gradation switch 207 is OFF, the charge amount Q flowing through the EL element is almost zero.

When the above operation is repeated at the frequency f, when the gradation switch 207 is ON, the amount of current I flowing through the EL element is
I = fQ = fC (V1-V2)
It becomes. When the gradation switch 207 is OFF, the amount of current I flowing through the EL element is zero. From this, the amount of current flowing through the EL element is the state of the gradation switch, the potential difference between the charging terminal 104 and the capacitor line 102 is V1, the potential difference between both ends of the discharged capacitor element 101 is V2, the frequency f, and the capacitance. It is determined by the capacitance C of the element 101. In order to obtain a certain light emission luminance at the maximum gradation, a current value I that must be passed through the EL element in order to emit light at that luminance at the maximum gradation is obtained, and the capacitance C and voltage of the capacitor 101 are obtained therefrom. The values V1, V2 and the frequency f are determined.

  The capacitance C of the capacitive element 101 is fixed to a certain constant value depending on the element characteristics. The potential difference V2 between both ends of the capacitive element 101 after discharge is determined by element characteristics such as a threshold voltage of the EL element and a discharge time depending on the frequency f. Here, when the frequency f is fixed, the potential difference V2 between both ends of the capacitive element 101 after the discharge is also fixed to a certain constant value.

  That is, in this embodiment, the value of the current flowing through the EL element can be determined by the state of the gradation switch if the frequency f and the potential difference V1 between the charging terminal 104 and the capacitor line 102 are fixed.

  The first to third states may be alternately repeated, or the second state and the third state may be repeated, and the first state may be inserted every certain period.

  Next, a case where FIG. 2B is used as the gradation means is shown. When FIG. 2B is used, an operation of repeating four states is performed.

  The first state is intended to write light emission and non-light emission signals into the gradation capacitor 210 as a signal for controlling the gradation of the EL element. The second state is intended to charge the capacitive element 101. The third state aims at discharging from the capacitive element 101 to the EL element. The fourth state is intended to write a voltage that causes the EL element to emit no light into the gradation capacitor 210.

  The first to third states are the same as those already described in the present embodiment.

  The fourth state is applied when the erase switch 212 shown in FIG. In the fourth state, the erase switch 212 is turned on, and a voltage that turns off the gradation switch 207 from the erase line 213 is applied to the gradation capacitor 210. When the erase switch 212 is turned off, the voltage for turning off the gradation switch 207 is held in the gradation capacitance.

  The first to fourth states may be alternately repeated, the second state and the third state may be repeated, and the first and fourth states may be inserted at certain intervals.

  2A and 2B, which is the gradation means, is that the erase switch 212 is provided in FIG. With the erase switch 212, the gradation switch 207 can be turned off regardless of the state of the write switch 208 or the signal line 209. In particular, when the number of gradations is large, the time for the subframe is shortened, so the time from when the gradation switch 207 is turned on to when it is turned off is shortened. Here, if there is a dedicated control circuit for turning off the gradation switch 207, it can be turned off easily even in a short time.

  FIGS. 4A and 4B show a method of controlling the light emission luminance by the gradation means. In the present embodiment, the light emission luminance is controlled according to the ON / OFF time of the gradation switch 207 shown in FIGS. 2A and 2B, and gradation display is performed.

  FIG. 4A shows an example of gradation display. When one second consists of 60 frames, the time per frame is 1/60 seconds. The light emission luminance is controlled by further dividing one frame according to the number of gradations and changing the light emission time. The number of divisions is 3 for 3-bit gradation. In this specification, each area of the divided frame is referred to as a subframe. The subframe has a time proportional to the weighting of each bit, and the luminance for each frame is determined by light emission and non-light emission of the subframe. In the case of 3 bits, when 1 bit is 1 (2 to the 0th power), 2 bits are 2 (2 to the 1st power) and 3 bits are 4 (2 to the 2nd power).

  FIG. 4B shows the relationship between the light emission and non-light emission of the subframe and the light emission luminance in the case of 8 gradation display. When all subframes emit light, the maximum light emission luminance is obtained. When all the subframes are not emitting light, the emission luminance is 0/8 of the maximum emission luminance. In the case of halftone, one subframe emits light, and another subframe does not emit light.

  Note that the subframe may be further finely divided without changing the number of gradations.

  The first and fourth states are inserted in the repetition of the second state and the third state for each subframe. Further, when the subframe is further divided, the subframe is inserted in the repetition of the second state and the third state for each division. Further, the first and fourth states may be further increased and inserted at the same frequency as the second and third states at the maximum.

  In the present embodiment, a current source is realized in the pixel by performing voltage-current conversion using the capacitive element 101, and gradation is expressed by the ON / OFF time ratio of the gradation switch. Since it can be driven by voltage from outside the pixel, the driving circuit outside the pixel is simple and does not need to be as complicated as a driving circuit having a current source.

  Further, when a voltage is applied to the pixel from the driver circuit, the signal reaches the pixel with a delay time according to the time constant of the wiring. Since the delay time is not affected by the voltage value, even if the voltage is very small, the signal reaches the pixel in a short time as in the case of the high voltage, so that halftone display close to black is possible and display quality does not deteriorate.

  Furthermore, by realizing the gradation expression by controlling the light emission time, only two kinds of voltages, ON and OFF, are required for the signal line voltage. This has the advantage of simplifying the power supply of the signal line.

  In general, in order to control the current value of an EL element, a TFT connected in series with the EL element is driven in a saturation region to supply current. Since the saturation region of the TFT is used, the power supply voltage is increased and the power consumption is increased. Also, the heat generation is large. In the present embodiment, a capacitive element is used to control the current value, so that power consumption can be kept low and heat generation can be reduced as compared with the case where a TFT is used.

  In the present embodiment, a configuration different from the configuration shown in the present embodiment may be used. In the first state, it is sufficient that the first electrode of the gradation capacitor 210 has a voltage equal to that of the signal line 209 and the second electrode has a fixed potential. In the second state, the first electrode of the capacitor 101 is electrically connected to the charging terminal 104, the second electrode is electrically connected to the capacitor line 102, and the charging terminal 104 and the capacitor line 102 are fixed differently. Any potential may be used. In the third state, the first electrode of the capacitor 101 is electrically connected to the EL element, the second electrode is electrically connected to the capacitor line 102, and the capacitor line 102 only needs to be a fixed potential. Further, the first electrode or the second electrode of the capacitor 102 may be in a floating state in the second state or the third state in accordance with the charging voltage of the capacitor 210. In the fourth state, the first electrode of the gradation capacitor 210 has the same voltage as the erase line 213, the second electrode has a fixed potential, and the erase line 213 is the first electrode of the capacitor 102 or the first electrode. Any potential may be used as long as the second electrode is in a floating state.

(Embodiment 3)
1 and 5 show another embodiment of the present invention. In the present embodiment, charging and discharging of the capacitive element are repeated by using charging means and discharging means, and the amount of charge charged to the capacitive element is changed by the gradation means in accordance with the light emission luminance, thereby causing a current to flow to the EL element. The purpose is to obtain a desired luminance.

  In this embodiment, the charging means and discharging means are the same as those shown in FIG. Further, since the configuration has already been shown, it will be omitted.

  FIG. 5 shows a gradation means for controlling the light emission gradation of the EL element.

  A gradation TFT 507, a writing switch 508, a signal line 509, a gradation capacitor 510, and a gradation capacitor line 511 are provided. P and P ′ are terminals inserted at positions d and e shown in FIG.

  The first and second electrodes of the gradation TFT 507 are inserted and connected to one of the positions d and e in FIG. 1, and the gate is the second electrode of the write switch 508 and the first of the gradation capacitor 510. It is connected to the electrode. A first electrode of the writing switch 508 is connected to the signal line 509. A second electrode of the gradation capacitor 510 is connected to the gradation capacitor line 511.

  The voltage of the gradation capacitor line 511 is not particularly limited as long as it is a fixed potential.

  A voltage corresponding to a gray scale is applied to the signal line 509, the amount of charge charged in the capacitor 101 is controlled, the luminance of the EL element is controlled, and the gray scale is expressed.

  The method of inserting the gradation means is the same as that of the second embodiment and is omitted because it is shown in FIG.

  The operation will be described. In this embodiment, the operation of repeating three states is performed.

  The first state is intended to write a luminance signal to the gradation capacitor 510 as a signal for controlling the gradation of the EL element. The second state is intended to charge the capacitive element 101. The third state aims at discharging from the capacitive element 101 to the EL element.

  In the first state, the writing switch 508 is turned ON, and a voltage corresponding to the gradation is applied to the gate of the gradation TFT 507 on the signal line 509. When the write switch 508 is turned off, the voltage of the signal line 509 is held in the gradation capacitor 510.

In the second state, the charge switch 103 is turned on and the discharge switch 105 is turned off. At this time, a current that has passed through the gradation TFT 507 flows from the charging terminal 104 to the capacitor 101. When the potential difference between the gate of the gradation TFT 507 and the capacitor line 102 is Vp, and the threshold voltage of the gradation TFT 507 is Vth, the voltage V1 applied to both ends of the capacitor 101 is
V1 = Vp-Vth
It becomes. However, the potential difference Va between the charging terminal 104 and the capacitor line 102 is
Va ≧ Vp−Vth
And That is, the voltage V1 across the capacitor 101 is determined according to the voltage Vp applied to the gate of the gradation TFT 507 and the threshold voltage Vth of the gradation TFT 507. The charge amount Q1 charged by the capacitive element 101 is C, where the capacitance of the capacitive element 101 is C.
Q1 = CV1 = C (Vp−Vth)
It becomes.

  In the third state, the charge switch 103 is turned off and the discharge switch 105 is turned on. At this time, the charge stored in the capacitor 101 flows from the discharge terminal 106 to the EL element.

When the potential difference between both ends of the capacitive element 101 after discharge is V2 (V1> V2), the amount of charge Q2 remaining in the capacitive element 101 after discharge is
Q2 = CV2
In the end, the amount of charge Q discharged and flowing to the EL element is
Q = Q1-Q2 = C (V1-V2) = C (Vp-Vth-V2)
It becomes.

When the above operation is repeated at the frequency f, the amount of current I flowing through the EL element is
I = fQ = fC (Vp−Vth−V2)
It becomes. Therefore, the amount of current flowing to the EL element is as follows: the voltage Vp applied to the gate of the gradation TFT 507, the threshold voltage of the gradation TFT 507 is Vth, the potential difference between both ends of the capacitive element 101 after discharge is V2, and the frequency f And the capacitance C of the capacitor 101.

  The capacitance C of the capacitor 101 and the threshold voltage Vth of the gradation TFT 507 are fixed to a certain constant value depending on the element characteristics. The potential difference V2 between both ends of the capacitive element 101 after discharge is determined by the element characteristics of the EL element and the discharge time depending on the frequency f. Here, when the frequency f is fixed, the potential difference V2 between both ends of the capacitive element 101 after the discharge is also fixed to a certain constant value.

  That is, in this embodiment, the value of the current flowing through the EL element can be determined by the voltage Vp applied to the gate of the gradation TFT 507. Since the voltage Vp applied to the gate of the gradation TFT 507 is equal to the voltage of the signal line 509 when the writing switch 508 is ON, the current value flowing through the EL element can be controlled by the voltage of the signal line 509. is there.

  By the way, when a current value is directly applied to a pixel as a means for driving an EL element whose luminance is determined in proportion to the current, the external drive circuit or the internal drive circuit becomes complicated, and a halftone display close to black Tends to be incomplete.

  In the method of controlling the current value by connecting the EL element and the TFT in series and changing the gate voltage of the TFT, the relationship between the gate voltage and the drain current of the TFT is a non-linear characteristic. It is necessary to control the relationship between voltage and voltage value nonlinearly. This complicates the external drive circuit or the internal drive circuit. Further, since it is difficult to make the TFT characteristics uniform, a gradation shift occurs.

  In the present embodiment, the luminance can be controlled by voltage, and the relationship between the voltage value and the luminance is linear. This is because the amount of current I flowing through the EL element is proportional to the voltage Vp applied to the gate of the gradation TFT 407. For this reason, the present embodiment has an advantage that the external drive circuit or the internal drive circuit is simple, no gradation shift occurs, and halftone display close to black is complete.

  In general, in order to control the current value of an EL element, a TFT connected in series with the EL element is driven in a saturation region to supply current. Since the saturation region of the TFT is used, the power supply voltage is increased and the power consumption is increased. Also, the heat generation is large. In the present embodiment, a capacitive element is used to control the current value, so that power consumption can be kept low and heat generation can be reduced as compared with the case where a TFT is used.

  In the present embodiment, a configuration different from the configuration shown in the present embodiment may be used. In the first state, it is only necessary that the first electrode of the gradation capacitor 510 has the same voltage as the signal line 509 and the second electrode has a fixed potential. In the second state, the first electrode of the capacitor 101 is connected to the first electrode of the gradation TFT 507, the second electrode is electrically connected to the capacitor line 102, and the second electrode of the gradation TFT 507 is connected. Is electrically connected to the charging terminal 104, the charging terminal 104 and the capacitor line 102 have different fixed potentials, the gate of the gradation TFT 507 is electrically connected to the first electrode of the gradation capacitor 510, and the gradation capacitance The second electrode 510 may be a fixed potential. In the third state, the first electrode of the capacitor 101 is electrically connected to the EL element, the second electrode is electrically connected to the capacitor line 102, and the capacitor line 102 only needs to be a fixed potential.

(Embodiment 4)
3 shows another embodiment of the present invention. In the present embodiment, charging and discharging of the capacitor element are repeated by using the charging unit and the discharging unit, a constant current is applied to the EL element, and the time for supplying the current according to the light emission gradation is changed by the gradation unit. An object of the present invention is to add the threshold voltage of the EL element to the charging voltage by the EL threshold correction means to obtain a desired luminance constant regardless of the deterioration of the EL element.

  In this embodiment, the charging means and discharging means are the same as those shown in FIG. Further, since the configuration has already been shown, it will be omitted.

  In this embodiment, the gradation means is the same as that shown in FIGS. 2 (A) and 2 (B). Further, since the configuration has already been shown, it will be omitted.

  FIG. 6 shows the EL threshold value correcting means. The EL threshold value correcting means corrects variations in threshold values of EL elements and the influence of deterioration. It has an EL threshold capacitance 621, a charging bias terminal 622, an EL threshold application switch 623, an EL threshold application TFT 624, an EL threshold acquisition switch 625, and an EL threshold acquisition terminal 626. Q and Q ′ are terminals to be inserted at the positions d and e shown in FIG.

  The first and second electrodes of the EL threshold application TFT 624 are inserted and connected to either d or e in FIG. 1, and the gate is connected to the first electrode of the EL threshold application switch 623. The second electrode of the EL threshold application switch 623 is connected to the first electrode of the EL threshold capacitor 621 and the second electrode of the EL threshold take-in switch 625. A second electrode of the EL threshold capacitor 621 is connected to the charging bias terminal 622. The first electrode of the EL threshold value capture switch 625 is connected to the EL threshold value capture terminal 626. The EL threshold value taking-in terminal 626 is connected to one place of a to d in FIG.

  When the gradation means and the EL threshold correction means are inserted at the same position, the gradation means and the threshold correction means are connected in series. In addition, when connecting in series, there is no particular limitation as to which one is connected to a position electrically close to the charging terminal.

  FIG. 7 shows an example in which gradation means is inserted. In the example, the gradation means in FIG. 2A and the EL threshold value correction means in FIG. 6 are inserted at the position indicated by e in FIG. Further, the EL threshold value capturing terminal of the EL threshold value correcting means in FIG. 6 is connected to the position indicated by a in FIG.

  The operation will be described. First, a case where FIG. 2A is used as the gradation means is shown. When FIG. 2A is used, an operation of repeating four states is performed.

  The first state is intended to write light emission and non-light emission signals into the gradation capacitor 210 as a signal for controlling the gradation of the EL element. The second state is intended to charge the capacitive element 101. The third state aims at discharging from the capacitive element 101 to the EL element. The fourth state aims at obtaining the threshold voltage of the EL element.

  Since the first state is the same as the first state of the second embodiment, a description thereof will be omitted.

  In the second state, the operation of the gradation means is the same as in the second state of the second embodiment. However, since the EL threshold correction means is inserted, the voltage applied to both ends of the capacitor 101 is different from that in the second embodiment.

  In the second state, as the operation of the EL threshold correction means in addition to the operation of the gradation means, the EL threshold take-in switch 625 is turned off, and a desired voltage under the condition that the EL threshold application TFT 624 is turned on is applied to the charging bias terminal 622. Then, the EL threshold application switch 623 is turned ON. At this time, if the insertion position of the first and second electrodes PP ′ of the gradation switch 207 is either a or b or c to f in FIG. Charging starts. Further, as the electric charge is charged, the potential of the first electrode of the capacitor 101 increases, and the gate-source voltage of the EL threshold application TFT 624 decreases. When the gate-source voltage of the EL threshold application TFT 624 falls below the threshold voltage of the EL threshold application TFT 624, the EL threshold application TFT 624 is turned off and charging is stopped.

When the potential of the charging bias terminal 622 is Vb and the potential difference between both ends of the EL threshold capacitor 621 is Velth (the potential of the first electrode ≧ the potential of the second electrode) when the cathode potential of the EL element is the reference potential, the EL The voltage applied to the gate of the threshold application TFT 624 is Vb + Velth. Further, when the threshold voltage of the EL threshold application TFT 624 is Vth, the potential of the first electrode of the capacitor 101 when the charging of the capacitor 101 is stopped is Vb + Velth−Vth. At this time, the amount of charge Q1 charged in the capacitive element 101 is Vg as the potential of the capacitive line 102 and C as the electrostatic capacitance.
Q1 = C (Vb + Velth−Vth−Vg)
It becomes. In addition, if the insertion positions of the first and second electrodes PP ′ of the gradation switch 207 are cf in FIG. 1 and the gradation switch 207 is OFF, there is no charge charging.

  In the third state, the operation of the gradation means is the same as in the third state of the second embodiment. However, EL threshold correction means is inserted, and the threshold voltage of the EL element is charged to the EL threshold capacity.

  In the third state, the charge switch 103 is turned off and the discharge switch 105 is turned on. At this time, the insertion position of the first and second electrodes PP ′ of the gradation switch 207 is either a to c or f in FIG. 1 and the gradation switch 207 is ON, or e or d. If there is, the charge stored in the capacitor 101 flows from the discharge terminal 106 to the EL element.

  If the insertion position of the first and second electrodes PP ′ of the gradation switch 207 is any of a to c or f in FIG. 1 and the gradation switch 207 is OFF, there is no charge discharge.

  Discharging stops when the voltage of the first electrode of the capacitor 101 becomes substantially equal to the threshold voltage of the EL element.

When the gradation switch 207 is ON, if the potential of the first electrode of the capacitor 101 after discharge is V2, and the potential of the capacitor line 102 is Vg, the amount of charge Q2 remaining in the capacitor 101 after discharge is ,
Q2 = C (V2-Vg)
In the end, the amount of charge Q discharged and flowing to the EL element is
Q = Q1-Q2 = C (Vb + Velth-Vth-V2)
It becomes. Here, since the potential of the first electrode of the capacitor 101 after discharge is equal to the threshold voltage of the EL element,
V2 = Velth
The amount of charge Q that is eventually discharged and flows to the EL element is
Q = C (Vb−Vth)
It becomes.

  When the gradation switch 207 is OFF, the amount of charge flowing through the EL element is almost zero.

  In the fourth state, the potential of the charging bias terminal 622 becomes the same potential as the cathode voltage of the EL element, the EL threshold application switch 623 is turned off, and the EL threshold take-in switch 625 is turned on. The discharge from the capacitor 101 to the EL element stops when the potential difference between the discharge terminal 106 and the cathode of the EL element reaches the threshold voltage of the EL element. At this time, the EL threshold value capture terminal 626 has the same potential as the discharge terminal 106. Further, the potential difference between both ends of the EL threshold capacitor 621 becomes the same as the threshold voltage of the EL element.

  That is, the threshold voltage of the EL element charged at both ends of the EL threshold capacitor 621 is charged by being added to Vb as a voltage expressed as Velth in the second state.

When the second state and the third state are repeated at the frequency f, when the gradation switch is ON, the current amount I flowing through the EL element is
I = fQ = fC (Vb−Vth)
It becomes. When the gradation switch is OFF, the current amount I flowing through the EL element is
I = 0
It becomes. From this, the amount of current flowing through the EL element includes the potential of the charging bias terminal 622 when the cathode potential of the EL element is the reference potential, Vb, the threshold voltage Vth of the EL threshold application TFT 624, the frequency f, and the capacitive element 101. , And the state of the gradation switch 207. In order to obtain a certain light emission luminance at the maximum gradation, a current value I that must be passed through the EL element in order to emit light at the maximum gradation is obtained, from which voltage values Vb, Vth, frequency f, capacitance The capacitance C of the element 101 is determined. However, since the capacitance C of the capacitive element 101 and the threshold voltage Vth of the EL threshold application TFT 624 are fixed as element characteristics, the actual determination is that the charging bias terminal 622 is obtained when the cathode potential of the EL element is used as the reference potential. Are the potential Vb and the frequency f.

  When the frequency f is high, the voltage of the first electrode of the capacitive element 101 after discharge may deviate from the threshold voltage of the EL element. However, even in that case, the potential difference between both ends of the EL threshold capacitance 621 is equal to the voltage of the first electrode of the capacitive element 101 after discharge, and the correction does not shift.

  In general, an EL element deteriorates when it emits light, and the threshold voltage changes. This is because, in a display device in which pixels that drive EL elements are arranged in a matrix, when the deterioration speed of the EL elements varies depending on the pixels, a display defect such as uneven brightness with different emission luminance occurs depending on the pixels. In this embodiment, the EL threshold voltage is added to the charging voltage to realize display that is not affected by the fluctuation of the EL threshold voltage.

  The first to fourth states are executed as follows. First, the first state is executed, the second state and the third state are alternately repeated at a frequency f, and the fourth state is executed simultaneously with the third state according to the time during which the EL threshold capacitor 621 can hold the charge. The In the first state, the gradation capacitor 210 is charged with a voltage corresponding to the light emission luminance of the EL element, the EL element is caused to emit light in the second state and the third state, and the threshold voltage of the EL element is set to the EL threshold capacity 621 in the fourth state. To charge. However, the first to fourth states may be alternately repeated.

  Next, a case where FIG. 2B is used as the gradation means is shown. When FIG. 2B is used, an operation of repeating five states is performed.

  The first state is intended to write light emission and non-light emission signals into the gradation capacitor 210 as a signal for controlling the gradation of the EL element. The second state is intended to charge the capacitive element 101. The third state aims at discharging from the capacitive element 101 to the EL element. The fourth state aims at obtaining the threshold voltage of the EL element. The purpose of the fifth state is to write a voltage that causes the EL element to emit no light into the gradation capacitor 210.

  The first to fourth states are the same as those already described in the present embodiment. In addition, the fifth state is the same as the fourth state of the second embodiment, and will be omitted.

  The first to fifth states are executed as follows. First, the first state is executed, the second state and the third state are alternately repeated at a frequency f, and the fourth state is executed simultaneously with the third state according to the time during which the EL threshold capacitor 621 can hold the charge. The fifth state is executed according to the gradation. In the first state, the gradation capacitor 210 is charged with a voltage corresponding to the light emission luminance of the EL element, the EL element is caused to emit light in the second state and the third state, and the threshold voltage of the EL element is set to the EL threshold capacity 621 in the fourth state. In the fifth state, the light emission time of the EL element is determined. However, the first to fifth states may be alternately repeated.

  In this embodiment, when the third state and the fourth state are executed simultaneously, the EL threshold voltage 621 can be charged to the EL threshold capacitor 621 every time the third state is entered. Even in this case, there is no problem because the EL threshold voltage once charged is always corrected to the latest EL threshold voltage. However, the EL threshold voltage does not change much in one charge / discharge time (10 microseconds if the frequency is 100 kHz). Therefore, it is not necessary to execute the fourth state every time the third state is entered. For example, the fourth state may be executed every frame time (1/60 seconds for 60 frames per second).

  In this embodiment, the EL threshold application switch 623 is not always necessary. However, when the gate parasitic capacitance of the EL threshold application TFT is large, it is desirable to attach an EL threshold application switch 623 to accurately reflect the EL threshold voltage.

  In the present embodiment, the potential of the charging bias terminal 622 is described as the same potential as the cathode voltage of the EL element in the third state, but the present invention is not limited to this. Even if the potential of the charging bias terminal 622 is an arbitrary potential, it is possible to correct the influence of variation and deterioration of the threshold voltage of the EL element.

  The present embodiment has the advantages described in the first and second embodiments in addition to correcting the influence due to the deterioration of the EL element.

  In the present embodiment, a configuration different from the configuration shown in the present embodiment may be used. In the first state, it is sufficient that the first electrode of the gradation capacitor 210 has a voltage equal to that of the signal line 209 and the second electrode has a fixed potential. In the second state, the first electrode of the capacitor 101 is electrically connected to the first electrode of the EL threshold application TFT 624, the second electrode is electrically connected to the capacitor line 102, and the EL threshold application TFT 624 is connected. The second electrode is electrically connected to the charging terminal 104, the charging terminal 104 and the capacitor line 102 are at different fixed potentials, and the gate of the EL threshold application TFT 624 is electrically connected to the first electrode of the EL threshold capacitor 621. Any potential may be used as long as the second electrode of the EL threshold capacitor 621 is connected to the charging bias terminal 622 and the charging bias terminal 622 turns on the EL threshold application TFT 624. In the third state, the first electrode of the capacitor 101 is electrically connected to the light-emitting element, the second electrode is electrically connected to the capacitor line 102, and the capacitor line 102 only needs to have a fixed potential. In the second state or the third state, the first electrode or the second electrode of the capacitor 102 may be in a floating state in accordance with the charging voltage of the capacitor 210 in the first state. In the fourth state, the first electrode of the EL threshold capacitor 621 may be electrically connected to the EL element and the second electrode may be a fixed potential. In the fifth state, the first electrode of the gradation capacitor 210 has the same voltage as the erase line 213, the second electrode has a fixed potential, and the erase line 213 is the first electrode of the capacitor 102 or the first electrode. Any potential may be used as long as the second electrode is in a floating state.

  Note that the EL threshold correction means shown in the present embodiment is also applicable to the first embodiment.

(Embodiment 5)
3 shows another embodiment of the present invention. In the present embodiment, charging and discharging of the capacitive element are repeated by using charging means and discharging means, and the amount of charge charged to the capacitive element is changed by the gradation means in accordance with the light emission luminance, thereby causing a current to flow to the EL element. Further, the EL threshold value correcting means adds the threshold voltage of the EL element to the charging voltage, and the desired luminance can be obtained regardless of the deterioration of the EL element.

  In this embodiment, the charging means and discharging means are the same as those shown in FIG. Further, since the configuration has already been shown, it will be omitted.

  FIG. 8 shows gradation means and EL threshold correction means. With the configuration shown in FIG. 8, gradation is expressed, and the influence of EL threshold deterioration is corrected by adding the threshold voltage of the EL element to the charging voltage. A gradation TFT 807, a writing switch 808, a signal line 809, a gradation capacitor 810, a gradation capacitor line 811, an EL threshold capacitor 821, an EL threshold application switch 823, an EL threshold take-in switch 825, And an EL threshold value taking-in terminal 826. Q and Q ′ are terminals to be inserted at the positions d and e shown in FIG.

  The first and second electrodes of the gradation TFT 807 are inserted and connected to either d or e in FIG. 1, and the gate is connected to the first electrode of the EL threshold application switch 823. The second electrode of the EL threshold application switch 823 is connected to the first electrode of the EL threshold capacitor 821 and the second electrode of the EL threshold take-in switch 825. The second electrode of the EL threshold capacitor 821 is connected to the second electrode of the writing switch 808 and the first electrode of the gradation capacitor 810. A first electrode of the writing switch 808 is connected to the signal line 809. A second electrode of the gradation capacitor 810 is connected to the gradation capacitor line 811. The first electrode of the EL threshold value capture switch 825 is connected to the EL threshold value capture terminal 826. The EL threshold value taking-in terminal 826 is connected to one place of a to d in FIG.

  The voltage of the gradation capacitor line 811 is not particularly limited as long as it is a fixed potential.

  A voltage corresponding to a gray scale is applied to the signal line 809, the amount of charge charged in the capacitor 101 is controlled, the luminance of the EL element is controlled, and the gray scale is expressed.

  The method for inserting the gradation means and the EL threshold value correcting means shown in FIG. 8 is the same as the method for inserting the EL threshold value correcting means in the sixth embodiment, and is omitted because it is shown in FIG.

  The operation will be described. In this embodiment, an operation of repeating four states is performed.

  The first state is intended to write a luminance signal to the gradation capacitor 510 as a signal for controlling the gradation of the EL element. The second state is intended to charge the capacitive element 101. The third state aims at discharging from the capacitive element 101 to the EL element. The fourth state aims at obtaining the threshold voltage of the EL element.

  In the first state, the writing switch 808 is turned on, and a voltage corresponding to the gradation is applied to the signal line 809. When the writing switch 808 is turned off, the voltage of the signal line 809 is held in the gradation capacitor 810.

  In the second state, the EL threshold value take-in switch 825 is turned off and the EL threshold value application switch 823 is turned on. Further, the charge switch 103 is turned on and the discharge switch 105 is turned off. At this time, a current that has passed through the gradation TFT 807 flows from the charging terminal 104 to the capacitor 101.

When the cathode potential of the EL element is the reference potential, the voltage of the first electrode of the gradation capacitor 810 is Vp, and the potential difference between both ends of the EL threshold capacitor 821 is Velth (first electrode potential ≧ second electrode potential). Potential), the gate voltage of the gradation TFT 807 is Vp + Velth. Further, when the threshold voltage of the gradation TFT 807 is Vth and the potential of the capacitor line 102 is Vg, the voltage V1 applied to both ends of the capacitor 101 is
V1 = Vp + Velth−Vth−Vg
It becomes. However, the potential difference Va between the charging terminal 104 and the capacitor line 102 is
Va ≧ Vp + Velth−Vth−Vg
And The charge amount Q1 charged by the capacitive element 101 is C, where the capacitance of the capacitive element 101 is C.
Q1 = CV1 = C (Vp + Velth−Vth−Vg)
It becomes.

In the third state, the charge switch 103 is turned off and the discharge switch 105 is turned on. At this time, the charge stored in the capacitor 101 flows from the discharge terminal 106 to the EL element. The EL threshold application switch 823 may be ON or OFF. When the potential of the first electrode of the capacitor 101 after discharge is V2 and the potential of the capacitor line 102 is Vg, the amount of charge Q2 remaining in the capacitor 101 after discharge is
Q2 = C (V2-Vg)
In the end, the amount of charge Q discharged and flowing to the EL element is
Q = Q1-Q2 = C (V1-V2 + Vg)
= C (Vp + Velth−Vth−V2)
It becomes. Here, the potential V2 of the first electrode of the capacitor 101 after discharging is a threshold voltage of the EL element. Further, when the potential difference Velth across the EL threshold capacitor 821 is the threshold voltage of the EL element,
V2 = Velth
It becomes. The amount of charge Q that eventually discharges and flows to the EL element is:
Q = C (Vb−Vth)
It becomes.

  In the fourth state, the potential of the signal line 809 becomes the same as that of the cathode of the EL element, the writing switch 808 is turned on, the EL threshold application switch 823 is turned off, and the EL threshold take-in switch 825 is turned on. Further, the charge switch 103 is turned off and the discharge switch 105 is turned on. The discharge from the capacitor 101 to the EL element stops when the potential difference between the discharge terminal 106 and the cathode of the EL element reaches the threshold voltage of the EL element. At this time, the EL threshold value capture terminal 826 has the same potential as the discharge terminal 106. Therefore, the potential difference between both ends of the EL threshold capacitor 821 is the same as the threshold voltage of the EL element. Therefore, in the third state, the potential difference Velth between both ends of the EL threshold capacitor 821 can be set as the threshold voltage of the EL element.

  The first to fourth states are executed as follows. First, the first state is executed, the second state and the third state are repeated at an alternate frequency f, and the fourth state is executed simultaneously with the third state. In the first state, the gradation capacitor 810 is charged with a voltage corresponding to the light emission luminance of the EL element, the EL element is caused to emit light in the second state and the third state, and the threshold voltage of the EL element is set to the EL threshold capacity 821 in the fourth state. To charge. However, the first to fourth states may be alternately repeated.

  The threshold voltage of the EL element charged at both ends of the EL threshold capacitor 821 is charged by being added to Vp as a voltage expressed as Velth in the second state.

When the second state and the third state are repeated at the frequency f, the current amount I flowing through the EL element is
I = fQ = fC (Vp−Vth)
It becomes. Therefore, the amount of current flowing through the EL element is determined by the potential difference Vp across the gradation capacitor 810, the threshold voltage Vth of the gradation TFT 507, the frequency f, and the capacitance C of the capacitor 101.

  The capacitance C of the capacitor 101 and the threshold voltage Vth of the gradation TFT 507 are fixed to a certain constant value depending on the element characteristics.

  That is, in this embodiment, the value of the current flowing through the EL element can be determined by the potential difference Vp across the gradation capacitor 810. Since this is equal to the voltage Vp of the signal line 809 when the writing switch 808 is ON, the current value flowing through the EL element can be controlled by the voltage of the signal line 809.

  In general, an EL element deteriorates when it emits light, and the threshold voltage changes. This is because, in a display device in which pixels that drive EL elements are arranged in a matrix, when the deterioration speed of the EL elements varies depending on the pixels, a display defect such as uneven brightness with different emission luminance occurs depending on the pixels. In this embodiment, the EL threshold voltage is added to the charging voltage to realize display that is not affected by the fluctuation of the EL threshold voltage.

  By the way, in the method of controlling the current value by connecting the EL element and the TFT in series and changing the gate voltage of the TFT, the relationship between the TFT gate voltage and the drain current is a non-linear characteristic. It is necessary to control the relationship between voltage and voltage value nonlinearly. This complicates the external drive circuit or the internal drive circuit. Further, since it is difficult to make the TFT characteristics uniform, a gradation shift occurs.

  In the present embodiment, the luminance can be controlled by voltage, and the relationship between the voltage value and the luminance is linear. This is because the amount of current I flowing through the EL element is proportional to the voltage Vp of the signal line 809. For this reason, the present embodiment has an advantage that the external drive circuit or the internal drive circuit is simple, gradation is not generated, and halftone display close to black is easy.

  In this embodiment, the potential of the signal line 809 is described as the same potential as the cathode voltage of the EL element in the fourth state, but the present invention is not limited to this. Even if the potential of the signal line 809 is an arbitrary potential, it is possible to correct the influence of variation and deterioration of the threshold voltage of the EL element.

  The present embodiment has the advantages described in the first and third embodiments in addition to correcting the influence due to the deterioration of the EL element.

  In the present embodiment, a configuration different from the configuration shown in the present embodiment may be used. In the first state, the first electrode of the gradation capacitor 810 has a voltage equal to that of the signal line 809 and the second electrode has a fixed potential. In the second state, the first electrode of the capacitor 101 is connected to the first electrode of the gradation TFT 807, the second electrode is electrically connected to the capacitor line 102, and the second electrode of the gradation TFT 807 is connected. Are electrically connected to the charging terminal 104, the charging terminal 104 and the capacitor line 102 have different fixed potentials, and an EL threshold capacitor 821 and a gradation capacitor 810 are provided between the gate of the gradation TFT 807 and the gradation capacitor line 811. It is only necessary that the gradation capacitor line 811 be electrically connected in series and have a fixed potential. In the third state, the first electrode of the capacitor 101 is electrically connected to the EL element, the second electrode is electrically connected to the capacitor line 102, and the capacitor line 102 only needs to be a fixed potential. In the fourth state, the first electrode of the EL threshold capacitor 821 may be electrically connected to the EL element and the second electrode may be a fixed potential.

(Embodiment 6)
3 shows another embodiment of the present invention. In the present embodiment, charging and discharging of the capacitor element are repeated by using the charging unit and the discharging unit, a constant current is applied to the EL element, and the time for supplying the current according to the light emission gradation is changed by the gradation unit. The threshold voltage of the EL element is added to the charging voltage by the EL threshold correction means, and the threshold voltage of the TFT is offset by the TFT threshold correction means, so that a desired brightness can be obtained regardless of the deterioration of the EL element or the variation of the TFT. With the goal.

  In this embodiment, the charging means and discharging means are the same as those shown in FIG. Further, since the configuration has already been shown, it will be omitted.

  In this embodiment, the gradation means is the same as that shown in FIGS. 2 (A) and 2 (B). Further, since the configuration has already been shown, it will be omitted.

  FIG. 9 shows EL threshold value correcting means and TFT threshold value correcting means. With the configuration shown in FIG. 9, the influence of EL threshold deterioration and TFT threshold variation is corrected by adding the threshold voltage of the EL element and TFT to the charging voltage. EL threshold capacitance 921, charging bias terminal 922, EL threshold application switch 923, EL threshold application TFT 924, EL threshold capture switch 925, EL threshold capture terminal 926, TFT threshold capacitance 931, and TFT threshold capacitance It includes a line 932, a TFT threshold first capture switch 941, a TFT threshold second capture switch 942, a TFT threshold third capture switch 943, and a TFT threshold fourth capture switch 944. Q and Q ′ are terminals to be inserted at the positions d and e shown in FIG.

  The first electrode of the EL threshold application TFT 924 is connected to the first electrode of the charge switch 103, the second electrode of the TFT threshold first take-in switch 941, and the first electrode of the TFT threshold second take-in switch 942. The second electrode is connected to the charging terminal 104 and the first electrode of the TFT threshold value third take-in switch 943 through the TFT threshold value fourth take-in switch 944, and the gate is connected to the TFT threshold value third take-in switch 943. The second electrode is connected to the first electrode of the TFT threshold capacitance 931. The second electrode of the TFT threshold capacitance 931 is connected to the first electrode of the EL threshold application switch 923 and the first electrode of the TFT threshold first take-in switch 941. The second electrode of the TFT threshold value second take-in switch 942 is connected to the TFT threshold capacity line 932. The second electrode of the EL threshold application switch 923 is connected to the first electrode of the EL threshold capacitor 921 and the second electrode of the EL threshold take-in switch 925. A second electrode of the EL threshold capacitance 921 is connected to the charging bias terminal 922. The first electrode of the EL threshold value capture switch 925 is connected to the EL threshold value capture terminal 926. The EL threshold value capture terminal 926 is connected to one of a to d shown in FIG.

  When the gradation means and the EL threshold correction means are inserted at the same position, the gradation means and the threshold correction means are connected in series. In addition, when connecting in series, there is no particular limitation as to which one is connected to a position electrically close to the charging terminal.

  FIG. 10 shows an example in which gradation means, EL threshold correction means, and TFT threshold correction means are inserted. In the example, the EL threshold value correcting means and the TFT threshold value correcting means of FIG. 9 are inserted at the positions indicated by Q and Q ′ at the position indicated by e in FIG. 1, and the EL threshold value capturing terminal 926 is connected at the position indicated by a. Similarly, the gradation means of FIG. 2A is inserted at the position indicated by a. Although the EL threshold value capture terminal 926 is connected to the terminal side indicated by P in FIG. 2A, it may be connected to the P ′ side.

  The operation will be described by taking the configuration of FIG. 10 as an example. In this embodiment, an operation of repeating five states is performed.

  The first state is intended to write light emission and non-light emission signals into the gradation capacitor 210 as a signal for controlling the gradation of the EL element. The second state is intended to charge the capacitive element 101. The third state aims at discharging from the capacitive element 101 to the EL element. The fourth state aims at obtaining the threshold voltage of the EL element. The fifth state aims at obtaining the threshold voltage of the EL threshold application TFT 924.

  Since the first state is the same as the first state of the second embodiment, a description thereof will be omitted.

  In the second state, the operation of the gradation means is the same as in the second state of the second embodiment. However, since the EL threshold correction unit and the TFT threshold correction unit are inserted, the voltage applied to both ends of the capacitor 101 is different from that in the second embodiment.

  In the second state, as the operation of the EL threshold correction means in addition to the operation of the gradation means, the EL threshold take-in switch 925 is turned off, and a desired voltage under the condition that the EL threshold application TFT 924 is turned on is applied to the charging bias terminal 922. Then, the EL threshold application switch 923 is turned on. As the operation of the TFT threshold value correcting means, the TFT threshold value first to third take-in switches 941 to 943 are turned off, and the TFT threshold value fourth take-in switch 944 is turned on. At this time, since the insertion position of the first and second electrodes PP ′ of the gradation switch 207 is at a position indicated by “a” in FIG. 1, charging of the capacitor element 101 is started. Further, as the charge is charged, the potential of the first electrode of the capacitor 101 increases, and the gate-source voltage of the EL threshold application TFT 924 decreases. When the voltage between the gate and source of the EL threshold application TFT 924 falls below the threshold voltage of the EL threshold application TFT 924, the EL threshold application TFT 924 is turned off and charging is stopped.

When the cathode potential of the EL element is the reference potential, the potential of the charging bias terminal 922 is Vb, the potential difference between both ends of the EL threshold capacitance 921 is Velth (the potential of the first electrode ≧ the potential of the second electrode), and the TFT threshold capacitance When the potential difference between both ends of 931 is Vtft (the potential of the first electrode ≧ the potential of the second electrode), the voltage applied to the gate of the EL threshold application TFT 924 is Vb + Velth + Vtft. Further, when the threshold voltage of the EL threshold application TFT 624 is Vth, the potential of the first electrode of the capacitor 101 when the charging of the capacitor 101 is stopped is Vb + Velth + Vtft−Vth. At this time, the amount of charge Q1 charged in the capacitive element 101 is Vg as the potential of the capacitive line 102 and C as the electrostatic capacitance.
Q1 = C (Vb + Velth + Vtft−Vth−Vg)
It becomes.

  The third state is the same as the third state of the second embodiment.

  In the fourth state, the potential of the charging bias terminal 922 is the same as the cathode voltage of the EL element, the EL threshold application switch 923 is turned off, and the EL threshold take-in switch 925 is turned on. The discharge from the capacitor 101 to the EL element stops when the potential difference between the discharge terminal 106 and the cathode of the EL element reaches the threshold voltage of the EL element. At this time, the EL threshold value capture terminal 926 is at the same potential as the discharge terminal 106. Further, the potential difference between both ends of the EL threshold capacitance 921 is the same as the threshold voltage of the EL element.

  That is, the threshold voltage of the EL element charged at both ends of the EL threshold capacitance 921 is charged with being added to Vb as a voltage expressed as Velth in the second state.

  If the gradation switch 207 is OFF, there is no charge discharge.

When the gradation switch 207 is ON, if the potential of the first electrode of the capacitor 101 after discharge is V2, and the potential of the capacitor line 102 is Vg, the amount of charge Q2 remaining in the capacitor 101 after discharge is ,
Q2 = C (V2-Vg)
It becomes.

  In the fifth state, the charging switch 103 is turned off, the EL threshold value application switch 923 is turned off, the TFT threshold value first to third take-in switches 941 to 943 are turned on, and the TFT threshold value fourth take-in switch 944 is turned off. In addition, the potential of the TFT threshold capacitance line 932 becomes lower than that of the charging terminal 104. At this time, the potential difference between the charging terminal 104 and the TFT threshold capacitance line 932 is equal to or higher than the threshold voltage of the EL threshold application TFT 924.

At this time, the potential difference between both ends of the TFT threshold capacitance 931 becomes equal to the threshold voltage Vth of the EL threshold application TFT 924. When the TFT threshold value first to third take-in switches 941 to 943 are turned OFF, the threshold voltage Vth of the EL threshold voltage application TFT 924 is held in the TFT threshold value capacitor 931. That is, in the second state, the potential difference between both ends of the TFT threshold capacitance 931 is equal to the threshold voltage Vth of the EL threshold application TFT 924, and the charge amount Q1 charged in the capacitive element 101 is
Q1 = C (Vb + Velth + Vtft−Vth−Vg)
= C (Vb + Velth-Vg)
It becomes. That is, the charge amount Q1 charged in the capacitor 101 is not affected by the threshold voltage Vth of the EL threshold application TFT 924.

  In the fifth state, the charging switch 103 may be ON.

After all, the amount of charge Q discharged and flowing to the EL element is
Q = Q1-Q2 = C (Vb + Velth-V2)
It becomes. Here, since the potential of the first electrode of the capacitor 101 after discharge is equal to the threshold voltage of the EL element,
V2 = Velth
The amount of charge Q that is eventually discharged and flows to the EL element is
Q = CVb
It becomes.

  When the gradation switch 207 is OFF, the amount of charge flowing through the EL element is almost zero.

When the above operation is repeated at the frequency f, when the gradation switch is ON, the amount of current I flowing through the EL element is
I = fQ = fCVb
It becomes. When the gradation switch is OFF, the current amount I flowing through the EL element is
I = 0
It becomes. From this, the amount of current flowing through the EL element includes the potential Vb of the charging bias terminal 822 when the cathode potential of the EL element is the reference potential, the frequency f, the capacitance C of the capacitor 101, and the gradation switch. And 207 state. In order to obtain a certain light emission luminance at the maximum gradation, a current value I that needs to be passed through the EL element to emit light at the maximum gradation is obtained, from which a voltage value Vb, a frequency f, and a capacitive element are obtained. The capacitance C of 101 is determined. However, since the capacitance C of the capacitive element 101 is fixed as an element characteristic, what is actually determined is the potential of the charging bias terminal 822 when the cathode potential of the EL element is the reference potential, Vb and the frequency f.

  Q and Q 'in FIG. 9 may be provided at the position d in FIG. In that case, the charging switch 103 is turned ON in the fifth state.

  In general, an EL element deteriorates when it emits light, and the threshold voltage changes. This is because, in a display device in which pixels that drive EL elements are arranged in a matrix, when the deterioration speed of the EL elements varies depending on the pixels, a display defect such as uneven brightness with different emission luminance occurs depending on the pixels. In this embodiment, the EL threshold voltage is added to the charging voltage to realize display that is not affected by the fluctuation of the EL threshold voltage.

  Moreover, it is difficult to make the TFT uniform characteristics due to the influence of its crystal state. This is because, in a display device in which pixels for driving EL elements are arranged in a matrix, the amount of charged charge varies depending on the pixel, and a display defect such as luminance unevenness occurs. In this embodiment, the threshold voltage of the EL threshold application TFT 924 is applied to the gate of the EL threshold application TFT 924 so that the charging voltage is not affected by the threshold voltage of the EL threshold application TFT 924. This realizes a display that is not affected by TFT non-uniformity.

  The first to fifth states are executed as follows. First, the first state is executed, the second state and the third state are alternately repeated at a frequency f, and the fourth state is executed simultaneously with the third state according to the time during which the EL threshold capacitor 921 can hold the charge. The fifth state is executed according to the time during which the TFT threshold capacitance 931 can hold the charge. In the first state, the gradation capacitor 210 is charged with a voltage corresponding to the light emission luminance of the EL element, the EL element is caused to emit light in the second state and the third state, and the threshold voltage of the EL element is set to the EL threshold capacity 921 in the fourth state. In the fifth state, the threshold voltage of the EL threshold application TFT 924 is charged to the TFT threshold capacitor 931. However, the first to fifth states may be alternately repeated.

  Next, a case where FIG. 2B is used as the gradation means is shown. When FIG. 2B is used, an operation of repeating six states is performed.

  The first to fifth states are the same as those already described in the present embodiment. Further, the sixth state is the same as the fourth state of the second embodiment, and is omitted.

  The first to sixth states are executed as follows. First, the first state is executed, the second state and the third state are alternately repeated at a frequency f, and the fourth state is executed simultaneously with the third state according to the time during which the EL threshold capacitor 921 can hold the charge. The fifth state is executed in accordance with the time during which the TFT threshold capacitance 931 can hold the charge, and the sixth state is executed in accordance with the gradation. In the first state, the gradation capacitor 210 is charged with a voltage corresponding to the light emission luminance of the EL element, the EL element is caused to emit light in the second state and the third state, and the threshold voltage of the EL element is set to the EL threshold capacity 921 in the fourth state. In the fifth state, the threshold voltage of the EL threshold application TFT 924 is charged to the TFT threshold capacitor 931, and in the sixth state, the light emission time of the EL element is determined. However, the first to sixth states may be alternately repeated.

  In this embodiment, in the third state, the EL threshold voltage is charged to the EL threshold capacitor 921 each time the third state is reached by always turning on the EL threshold value taking switch. Even in this case, there is no problem because the EL threshold voltage once charged is always corrected to the latest EL threshold voltage. However, the EL threshold voltage does not change much in one charge / discharge time (10 microseconds if the frequency is 100 kHz). Therefore, it is not necessary to charge the EL threshold capacitor 921 with the EL threshold voltage every time the third state is entered. For example, the EL threshold voltage may be charged to the EL threshold capacitor 921 every frame time (1/60 seconds for 60 frames per second). In this case, when the EL threshold capacitance 921 is not charged, the EL threshold take-in switch 925 remains OFF, and the voltage of the charging bias terminal 922 is arbitrary.

  In the present embodiment, the potential of the charging bias terminal 922 is described as the same potential as the cathode voltage of the EL element in the third state, but the present invention is not limited to this. Even if the potential of the charging bias terminal 922 is an arbitrary potential, it is possible to correct the influence of variation and deterioration of the threshold voltage of the EL element.

  This embodiment has the advantages shown in the first, second, and fourth embodiments in addition to correcting the influence of TFT non-uniformity.

  In the present embodiment, a configuration different from the configuration shown in the present embodiment may be used. In the first state, it is sufficient that the first electrode of the gradation capacitor 210 has a voltage equal to that of the signal line 209 and the second electrode has a fixed potential. In the second state, the first electrode of the capacitor 101 is electrically connected to the first electrode of the EL threshold application TFT 924, the second electrode is electrically connected to the capacitor line 102, and the EL threshold application TFT 924 is connected. The second electrode is electrically connected to the charging terminal 104, the charging terminal 104 and the capacitor line 102 have different fixed potentials, and the EL threshold capacitance 921 is connected between the gate of the EL threshold application TFT 924 and the charging bias terminal 922. Any potential may be used as long as it is electrically connected in series with the TFT threshold capacitance 931 and the charging bias terminal 622 turns on the EL threshold application TFT 924. In the third state, the first electrode of the capacitor 101 is electrically connected to the light-emitting element, the second electrode is electrically connected to the capacitor line 102, and the capacitor line 102 only needs to have a fixed potential. In the second state or the third state, the first electrode or the second electrode of the capacitor 102 may be in a floating state in accordance with the charging voltage of the capacitor 210 in the first state. In the fourth state, the first electrode of the EL threshold capacitance 921 may be electrically connected to the EL element and the second electrode may be a fixed potential. In the fifth state, the first electrode and the gate of the EL threshold application TFT 924 are kept at a certain potential, the second electrode is kept at a different potential, and the gate is connected to the first electrode of the TFT threshold capacitance 931. It is only necessary that the second electrode is connected to a fixed potential. In the sixth state, the first electrode of the gradation capacitor 210 has the same voltage as that of the erasing line 213, the second electrode has a fixed potential, and the erasing line 213 has the first electrode of the capacitor 102 or the first electrode. Any potential may be used as long as the second electrode is in a floating state.

(Embodiment 7)
3 shows another embodiment of the present invention. In the present embodiment, charging and discharging of the capacitive element are repeated by using charging means and discharging means, and the amount of charge charged to the capacitive element is changed by the gradation means in accordance with the light emission luminance, thereby causing a current to flow to the EL element. The threshold voltage of the EL element is added to the charging voltage by the EL threshold correction means, and the threshold voltage of the TFT is offset by the TFT threshold correction means, and the desired luminance is related to the deterioration of the EL element and the variation of the TFT. The aim is to get without.

  In this embodiment, the charging means and discharging means are the same as those shown in FIG. Further, since the configuration has already been shown, it will be omitted.

  FIG. 11 shows gradation means, EL threshold correction means, and TFT threshold correction means. With the configuration shown in FIG. 11, the gradation is expressed, and the threshold voltage of the EL element and TFT is added to the charging voltage, thereby correcting the influence of EL threshold deterioration and TFT threshold variation. Gradation TFT 1107, writing switch 1108, signal line 1109, gradation capacitance 1110, gradation capacitance line 1111, EL threshold capacitance 1121, EL threshold application switch 1123, EL threshold capture switch 1125, The EL threshold value capture terminal 1126 includes a TFT threshold value capacitance 1131, a TFT threshold value capacitance line 1132, TFT threshold value first to third capture switches 1141 to 1143, and a TFT threshold value fourth capture switch 1144. Q and Q ′ are terminals to be inserted at the position e shown in FIG.

  The first electrode of the gradation TFT 1107 is connected to the first electrode of the charge switch 103, the second electrode of the TFT threshold value first take-in switch 1141, and the first electrode of the TFT threshold value second take-in switch 1142. The second electrode is connected to the charging terminal 104 and the first electrode of the TFT threshold third take-in switch 1143 via the TFT threshold fourth take-in switch 1144, and the gate is the second of the TFT threshold third take-in switch 1143. Are connected to the first electrode of the TFT threshold capacitance 1131. The second electrode of the TFT threshold capacitance 1131 is connected to the first electrode of the EL threshold application switch 1123 and the first electrode of the TFT threshold first take-in switch 1141. The second electrode of the TFT threshold second take-in switch 1142 is connected to the TFT threshold capacitance line 1132. The second electrode of the EL threshold application switch 1123 is connected to the first electrode of the EL threshold capacitance 1121 and the second electrode of the EL threshold take-in switch 1125. The second electrode of the EL threshold capacitor 1121 is connected to the first electrode of the gradation capacitor 1110 and the second electrode of the writing switch 1108. A second electrode of the gradation capacitor 1110 is connected to the gradation capacitor line 1111. A first electrode of the writing switch 1108 is connected to the signal line 1109. The first electrode of the EL threshold value capture switch 1125 is connected to the EL threshold value capture terminal 1126. The EL threshold value capture terminal 1126 is connected to one of a to d shown in FIG.

  The voltage of the gradation capacitor line 1111 is not particularly limited as long as it is a fixed potential.

  A voltage corresponding to a gray scale is applied to the signal line 1109, the amount of charge charged in the capacitor 101 is controlled, the luminance of the EL element is controlled, and the gray scale is expressed.

  The method of inserting the gradation means, the EL threshold correction means, and the TFT threshold correction means shown in FIG. 11 is the same as the method of inserting the EL threshold correction means and the TFT threshold correction means in the sixth embodiment, and in FIG. I omitted it.

The operation will be described. In this embodiment, an operation of repeating five states is performed.

  The first state is intended to write a luminance signal to the gradation capacitor 1110 as a signal for controlling the gradation of the EL element. The second state is intended to charge the capacitive element 101. The third state aims at discharging from the capacitive element 101 to the EL element. The fourth state aims at obtaining the threshold voltage of the EL element. The fifth state aims at obtaining the threshold voltage of the gradation TFT 1107.

  Since the first state is the same as the first state of the fifth embodiment, a description thereof will be omitted.

  In the second state, the EL threshold value capture switch 1125 is turned off, the EL threshold value application switch 1123 is turned on, the TFT threshold value first to third capture switches 1141 to 1143 are turned off, and the TFT threshold value fourth capture switch 1144 is turned on. Turn on. Further, the charge switch 103 is turned on and the discharge switch 105 is turned off. At this time, a current that has passed through the gradation TFT 1107 flows from the charging terminal 104 to the capacitor 101.

When the cathode potential of the EL element is a reference potential, the voltage of the first electrode of the gradation capacitor 1110 is Vp, and the potential difference between both ends of the EL threshold capacitor 1121 is Velth (first electrode potential ≧ second electrode potential). Potential), if the potential difference between both ends of the TFT threshold capacitance 1131 is Vtft (the potential of the first electrode ≧ the potential of the second electrode), the gate voltage of the gradation TFT 1107 is Vp + Velth + Vtft. When the threshold voltage of the gradation TFT 1107 is Vth and the potential of the capacitor line 102 is Vg, the voltage V1 applied to both ends of the capacitor 101 is
V1 = Vp + Velth + Vtft−Vth−Vg
It becomes. However, the potential difference Va between the charging terminal 104 and the capacitor line 102 is
Va ≧ Vp + Velth + Vtft−Vth−Vg
And The charge amount Q1 charged by the capacitive element 101 is C, where the capacitance of the capacitive element 101 is C.
Q1 = CV1 = C (Vp + Velth + Vtft−Vth−Vg)
It becomes.

In the third state, the charge switch 103 is turned off and the discharge switch 105 is turned on. At this time, the charge stored in the capacitor 101 flows from the discharge terminal 106 to the EL element. The EL threshold application switch 1123 may be ON or OFF. When the potential of the first electrode of the capacitor 101 after discharge is V2 and the potential of the capacitor line 102 is Vg, the amount of charge Q2 remaining in the capacitor 101 after discharge is
Q2 = C (V2-Vg)
In the end, the amount of charge Q discharged and flowing to the EL element is
Q = Q1-Q2 = C (V1-V2 + Vg)
= C (Vp + Velth + Vtft−Vth−V2)
It becomes. Here, the potential V2 of the first electrode of the capacitor 101 after discharging is a threshold voltage of the EL element. Further, when the potential difference Velth between both ends of the EL threshold capacitance 1121 is the threshold voltage of the EL element,
V2 = Velth
It becomes. The amount of charge Q that eventually discharges and flows to the EL element is:
Q = C (Vp + Vtft−Vth)
It becomes.

  In the fourth state, the potential of the signal line 1109 is the same as that of the cathode of the EL element, the writing switch 1108 is turned on, the EL threshold value application switch 1123 is turned off, and the EL threshold value taking switch 1125 is turned on. Further, the charge switch 103 is turned off and the discharge switch 105 is turned on. The discharge from the capacitor 101 to the EL element stops when the potential difference between the discharge terminal 106 and the cathode of the EL element reaches the threshold voltage of the EL element. At this time, the EL threshold value capture terminal 1126 has the same potential as the discharge terminal 106. Therefore, the potential difference between both ends of the EL threshold capacitance 1121 is the same as the threshold voltage of the EL element. Therefore, in the third state, the potential difference Velth across the EL threshold capacitor 1121 can be set as the threshold voltage of the EL element.

  In the fifth state, the charging switch 103 is turned off, the EL threshold application switch 1123 is turned off, the TFT threshold first to third take-in switches 1141 to 1143 are turned on, and the TFT threshold fourth take-in switch 1144 is turned off. Further, the potential of the TFT threshold capacitance line 1132 becomes lower than that of the charging terminal 104. At this time, the potential difference between the charging terminal 104 and the TFT threshold capacitance line 1132 is equal to or higher than the threshold voltage of the gradation TFT 1107.

At this time, the potential difference between both ends of the TFT threshold capacitance 1131 becomes equal to the threshold voltage Vth of the gradation TFT 1107. When the TFT threshold value first to third take-in switches 1141 to 1143 are turned OFF, the threshold voltage Vth of the gradation TFT 1107 is held in the TFT threshold capacitor 1131. In other words, the potential difference Vtft between the both ends of the TFT threshold capacitance 1131 shown in the second state is equal to the threshold voltage Vth of the EL threshold application TFT 1124. The amount of charge Q that eventually discharges and flows to the EL element is:
Q1 = C (Vp + Vtft−Vth) = CVp
It becomes. That is, the amount of charge Q1 charged in the capacitor 101 is determined by the voltage Vp input from the signal line 1109 in the first state without being affected by the threshold voltage of the EL element and the gradation TFT 1107.

  The first to fifth states are executed as follows. First, the first state is executed, the second state and the third state are repeated at an alternate frequency f, and the fourth state and the fifth state are executed simultaneously with the third state. In the first state, the gradation capacitor 1110 is charged with a voltage corresponding to the light emission luminance of the EL element, the EL element is caused to emit light in the second state and the third state, and the threshold voltage of the EL element is set to the EL threshold capacity 1121 in the fourth state. In the fifth state, the threshold voltage of the gradation TFT is charged into the TFT threshold capacitor 1131. However, the first to fifth states may be alternately repeated, and the fourth and fifth states may be executed irregularly. The fourth state can be executed at the same time as the third state, and the fifth state can be executed if the TFT threshold first to third intake switches 1141 to 1143 are ON and the TFT threshold fourth intake switch 1144 is ON. It is.

  The threshold voltage of the EL element charged at both ends of the EL threshold capacitor 1121 and the threshold voltage of the gradation TFT 1107 charged at both ends of the TFT threshold capacitor 1131 are expressed as Velth + Vtft in the second state with respect to Vp. Will be charged.

When the second state and the third state are repeated at the frequency f, the current amount I flowing through the EL element is
I = fQ = fCVp
It becomes. Therefore, the amount of current flowing through the EL element is determined by the potential difference Vp across the gradation capacitor 810, the frequency f, and the capacitance C of the capacitor 101.

  The capacitance C of the capacitive element 101 is fixed to a certain constant value depending on the element characteristics.

  That is, in this embodiment, the value of the current flowing through the EL element can be determined by the potential difference Vp across the gradation capacitor 1110. This is equal to the voltage Vp of the signal line 1109 when the writing switch 1108 is ON, so that the current value flowing through the EL element can be controlled by the voltage of the signal line 1109.

  11 may be provided at the position of d in FIG. In that case, the charging switch 103 is turned ON in the fifth state.

  By the way, it is difficult to make uniform characteristics of TFT due to its crystal state and the like. This is because, in a display device in which pixels for driving EL elements are arranged in a matrix, the amount of charged charge varies depending on the pixel, and a display defect such as luminance unevenness occurs. In this embodiment, the threshold voltage of the gradation TFT 1107 is applied on the gate of the gradation TFT 1107 so that the charging voltage is not affected by the threshold voltage of the gradation TFT 1107. This realizes a display that is not affected by TFT non-uniformity.

  In the method of controlling the current value by connecting the EL element and the TFT in series and changing the gate voltage of the TFT, the relationship between the gate voltage and the drain current of the TFT is a non-linear characteristic. It is necessary to control the relationship between voltage and voltage value nonlinearly. This complicates the external drive circuit or the internal drive circuit. Further, since it is difficult to make the TFT characteristics uniform, a gradation shift occurs.

  In the present embodiment, the luminance can be controlled by voltage, and the relationship between the voltage value and the luminance is linear. This is because the amount of current I flowing through the EL element is proportional to the voltage Vp of the signal line 1109. For this reason, the present embodiment has an advantage that the external drive circuit or the internal drive circuit is simple, no gradation shift occurs, and halftone display close to black is complete.

  In this embodiment, the potential of the signal line 1109 is described as the same potential as the cathode voltage of the EL element in the fourth state, but the present invention is not limited to this. Even if the potential of the signal line 1109 is an arbitrary potential, it is possible to correct the influence of variation and deterioration of the threshold voltage of the EL element.

  The present embodiment has the advantages shown in the first, third, and fifth embodiments in addition to correcting the influence of the non-uniformity of the TFT.

  In the present embodiment, a configuration different from the configuration shown in the present embodiment may be used. In the first state, the first electrode of the gradation capacitor 1110 has a voltage equal to that of the signal line 1109 and the second electrode has a fixed potential. In the second state, the first electrode of the capacitor 101 is electrically connected to the first electrode of the gradation TFT 1107, the second electrode is electrically connected to the capacitor line 102, and the second electrode of the gradation TFT 1107 is electrically connected. The second electrode is electrically connected to the charging terminal 104, the charging terminal 104 and the capacitor line 102 have different fixed potentials, and the gradation capacitor 1110 and the EL threshold value are provided between the gate of the gradation TFT 1107 and the gradation capacitor line 1111. It is sufficient that the capacitor 1121 and the TFT threshold capacitor 1131 are electrically connected in series and the gradation capacitor line 1111 has a fixed potential. In the third state, the first electrode of the capacitor 101 is electrically connected to the light-emitting element, the second electrode is electrically connected to the capacitor line 102, and the capacitor line 102 only needs to have a fixed potential. In the fourth state, the first electrode of the EL threshold capacitor 1121 may be electrically connected to the EL element and the second electrode may be a fixed potential. In the fifth state, the first electrode and the gate of the gradation TFT 1107 are kept at the same potential, the second electrode is kept at a different potential, and the gate is connected to the first electrode of the TFT threshold capacitance 1131. The second electrode of the TFT threshold capacitance 1131 only needs to be at a fixed potential.

  The TFT threshold value correcting means shown in this embodiment can also be applied to the third embodiment.

  Examples of the present invention will be described below.

[Example 1]
In this example, an example of a display device using the structure shown in Embodiment Modes 2, 4, and 6 for a pixel will be described. FIG. 12 shows a configuration example of the display device. In this embodiment, it is assumed that the gradation means uses the configuration having the erase switch shown in FIG.

  A plurality of pixels 1208 have a pixel portion 1207 arranged in a matrix of m rows and n columns, and around the pixel portion 1207 are a source scanning circuit 1201, a video latch circuit 1204, a write gate scanning circuit 1205, and an erase gate scanning. A circuit 1206 is included. Each pixel is connected to a signal line 1209 for each column, and the signal line 1209 is connected to a video latch circuit 1204. Each pixel is further connected to a write gate line 1210 and an erase gate line 1211 for each row, the write gate line 1210 is connected to a write gate scanning circuit 1205, and the erase gate line 1211 is connected to an erase gate scanning circuit 1206. Connected. The video latch circuit 1204 is connected to the source scanning circuit 1201 and the video signal line 1203. A video signal corresponding to image information is input to the video signal line 1203 via a video signal input terminal 1202.

  A signal line 1209 illustrated in FIG. 12 is the signal line 209 illustrated in FIG. Further, the write gate line 1210 and the erase gate line 1211 shown in FIG. 12 are connected to the control electrodes of the write switch 208 and the erase switch 212 shown in FIG.

  Note that control electrodes, various capacitor lines, terminals, and the like of other switch elements shown in the second, fourth, and sixth embodiments are omitted. Further, control signals and power sources for the source scanning circuit 1201, the write gate scanning circuit 1205, and the erase gate scanning circuit 1206 are also omitted.

  The operation will be described.

  A write gate start pulse and a row selection clock signal are input to the write gate scanning circuit 1205. The write gate scanning circuit 1205 sequentially scans from Gw1 to Gwm, and sequentially turns on the write switch 208 of the pixel 1208 via the write gate line 1210.

  A source start pulse and a column selection clock signal are input to the source scanning circuit 1201. While the write gate scanning circuit 1205 selects one row of the pixels 1208, the source scanning circuit 1201 scans in synchronization with the column selection clock, and sequentially selects the video latch circuit 1204 from S1 to Sn. At the same time, a video signal corresponding to the image information is input from the video signal input terminal 1202 and input to the video latch circuit 1204 through the video signal line 1203. The video signal is taken in and stored in the video latch circuit 1204 selected by the source scanning circuit 1201, and is output to the signal line 1209. When the above operation is executed from S1 to Sn of the video latch circuit 1204, a video signal for one row is output from the video latch circuit 1204 to the signal line 1209. At the same time, the corresponding video signal is input to one row of the pixels 1208 selected by the write gate scanning circuit 1205.

  The erasure gate scanning circuit 1206 turns on the erasure gate line 1211 after the time corresponding to the gradation has elapsed after the writing gate scanning circuit 1205 turns on the writing switch 208 of the pixel 1208 in the corresponding row to write the video signal. Then, the erase switch 212 of the pixel in the same row is turned on to erase the video signal.

  The period from the writing of the video signal at the timing of the writing gate scanning circuit 1205 to the erasing of the video signal at the timing of the erasing gate scanning circuit 1206 is the time during which the EL element of the pixel emits or does not emit light. From the erasing of the video signal at the timing of the erasing gate scanning circuit 1206 to the writing of the video signal at the timing of the writing gate scanning circuit 1205, the EL element of the pixel does not emit light.

  The video signal is a voltage for turning on or off the gradation switch 207 according to image information. Further, the voltage of a control signal that is not described in the present embodiment and is input to the control electrode of another switch element is also a voltage for turning on or off the switch element. Further, each terminal voltage may be a certain voltage or at most two voltages.

  A voltage that is turned ON or OFF, such as a video signal or a control signal, or a terminal that has only one or two potentials can be controlled by a digital signal. Since it can be controlled by a digital signal, a digital-analog conversion circuit is unnecessary.

  Further, in this embodiment, since the light emission luminance of the EL element is controlled by the light emission time, the luminance can be controlled more accurately than in the case of controlling by an analog current or voltage.

  In general, a current source is required to drive an EL element. However, the current source is particularly difficult to design and manufacture with TFT, the circuit scale is large, and the power consumption is high. In the present invention, by realizing voltage-current conversion by a capacitive element in a pixel, it can be driven by a voltage as viewed from the outside of the pixel. The fact that no current source is required is a great advantage of the present invention.

  Another major feature is that the deterioration and variation of the EL elements and TFTs are corrected despite being driven by voltage.

[Example 2]
In this example, an example of a display device using the structure described in Embodiment Modes 3, 5, and 7 for a pixel will be described. FIG. 13 shows a configuration example of the display device.

  A plurality of pixels 1308 includes a pixel portion 1307 arranged in a matrix of m rows and n columns, and a source scanning circuit 1301, a video signal switch 1304, and a write gate scanning circuit 1305 are provided around the pixel portion 1307. Yes. Each pixel is connected to a signal line 1309 for each column, and the signal line 1309 is connected to a video signal line 1303 via a video signal switch 1304. Each pixel is further connected to a write gate line 1310 for each row, and the write gate line 1310 is connected to a write gate scanning circuit 1305. The control electrode of the video signal switch 1304 is connected to the source scanning circuit 1301. A video signal corresponding to image information is input to the video signal line 1303 via a video signal input terminal 1202.

  The signal line 1309 shown in FIG. 13 is the signal line 509 shown in FIG. 5 in the third embodiment, the signal line 809 shown in FIG. 8 in the fifth embodiment, and the signal line 809 shown in FIG. 11 is a signal line 1109 shown in FIG. 13 is the write switch 508 shown in FIG. 5 in the third embodiment, the write switch 808 shown in FIG. 8 in the fifth embodiment, and the write switch 808 shown in the seventh embodiment. For example, it is connected to the control electrode of the write switch 1108 shown in FIG.

  Note that control electrodes, various capacitor lines, terminals, and the like of other switch elements shown in the third, fifth, and seventh embodiments are omitted. Further, the control signals and power supply for the source scanning circuit 1301 and the write gate scanning circuit 1305 are also omitted.

  The operation will be described.

  A write gate start pulse and a row selection clock signal are input to the write gate scanning circuit 1305. The write gate scanning circuit 1305 sequentially scans from G1 to Gm, and sequentially turns on the write switches 508, 808, or 1108 of the pixels 1308 via the write gate line 1310.

  A source start pulse and a column selection clock signal are input to the source scanning circuit 1301. During the period when the write gate scanning circuit 1305 selects one row of the pixels 1308, the source scanning circuit 1301 scans in synchronization with the column selection clock, and sequentially selects the control electrodes of the video signal switch 1304 from S1 to Sn. To go. At the same time, a video signal corresponding to the image information is input from the video signal input terminal 1302 and input to the first electrode of the video signal switch 1304 through the video signal line 1303. The video signal passes from the first electrode of the video signal switch 1304 selected by the source scanning circuit 1301 to the second electrode, and is output to the signal line 1309. When the above operation is performed from S1 to Sn of the video signal switch 1304, video signals for one row are output from the video signal switch 1304 to the signal line 1309. At the same time, a corresponding video signal is input to one row of the pixels 1308 selected by the write gate scanning circuit 1305.

  The video signal is an analog voltage proportional to the gradation. Further, the voltage of a control signal input to the control electrode of another switch element, which is not described in this embodiment, is a voltage for turning on or off the switch element. Furthermore, each terminal voltage is a fixed potential.

  A voltage that is turned ON or OFF, such as a control signal, or a terminal having a fixed potential can be controlled by a digital signal. Since it can be controlled by a digital signal, a digital-analog conversion circuit is unnecessary.

  In this embodiment, since writing to the pixel for controlling the light emission luminance of the EL element is a voltage value, writing can be performed at a higher speed than when writing with current. This increases the display quality because it can be accurately written even in a halftone close to black.

  In general, a current source is required to drive an EL element. However, the current source is particularly difficult to design and manufacture with TFT, the circuit scale is large, and the power consumption is high. In the present invention, by realizing voltage-current conversion by a capacitive element in a pixel, it can be driven by a voltage as viewed from the outside of the pixel. The fact that no current source is required is a great advantage of the present invention.

  In addition, since voltage-current conversion using TFTs is non-linear, in order to drive an EL element driven by current with voltage, processing corresponding to non-linear voltage-current conversion is necessary, and the circuit scale is large. The gradation display tends to be inaccurate. In the present invention, voltage-current conversion by a capacitive element in a pixel is realized, and voltage-current conversion in the capacitive element is linear, so that gradation control can be easily performed. In addition, gradation control using analog video signals as in the present embodiment is widely used in LCDs and the like, and its technology and components are easy to apply.

  Another major feature is that the deterioration and variation of the EL elements and TFTs are corrected despite being driven by voltage.

[Example 3]
The semiconductor device of the present invention has various uses. In this embodiment, examples of electronic devices to which the present invention can be applied will be described.

  Examples of such electronic devices include portable information terminals (electronic notebooks, mobile computers, mobile phones, etc.), video cameras, digital cameras, personal computers, televisions, and the like. An example of them is shown in FIG.

  FIG. 14A illustrates an EL display, which includes a housing 3301, a support base 3302, a display portion 3303, and the like. The display device of the present invention can be used in the display portion 3303.

  FIG. 14B illustrates a video camera, which includes a main body 3311, a display portion 3312, an audio input portion 3313, operation switches 3314, a battery 3315, an image receiving portion 3316, and the like. The display device of the present invention can be used in the display portion 3312.

  FIG. 14C illustrates a personal computer, which includes a main body 3321, a housing 3322, a display portion 3323, a keyboard 3324, and the like. The display device of the present invention can be used in the display portion 3323.

  FIG. 14D illustrates a portable information terminal which includes a main body 3331, a stylus 3332, a display portion 3333, operation buttons 3334, an external interface 3335, and the like. The display device of the present invention can be used in the display portion 3333.

  FIG. 14E illustrates a mobile phone, which includes a main body 3401, an audio output portion 3402, an audio input portion 3403, a display portion 3404, operation switches 3405, and an antenna 3406. The display device of the present invention can be used in the display portion 3404.

  FIG. 14F illustrates a digital camera, which includes a main body 3501, a display portion (A) 3502, an eyepiece portion 3503, operation switches 3504, a display portion (B) 3505, and a battery 3506. The display device of the present invention can be used in the display portion (A) 3502 and the display portion (B) 3505.

  Even when the semiconductor device of the present invention is an LSI or the like, it can be used for the electronic devices shown in FIGS.

  As described above, the applicable range of the present invention is so wide that it can be used for electronic devices in various fields.

FIG. 6 is a diagram showing a charging unit and a discharging unit in the semiconductor device of the present invention. FIG. 6 is a diagram showing gradation means in the semiconductor device of the present invention. An example in which gradation means is added to charging means and discharging means. An example in which gradation is controlled by light emission time. FIG. 6 is a diagram showing gradation means in the semiconductor device of the present invention. FIG. 4 is a diagram showing an EL threshold value correcting unit in the semiconductor device of the present invention. An example in which gradation means and EL threshold correction means are added to charging means and discharging means. FIG. 4 is a diagram showing a gradation unit and an EL threshold correction unit in the semiconductor device of the present invention. FIG. 6 is a diagram showing an EL threshold value correcting unit and a TFT threshold value correcting unit in the semiconductor device of the present invention. An example in which gradation means, EL threshold correction means, and TFT threshold correction means are added to charging means and discharging means. FIG. 4 is a diagram illustrating a gradation unit, an EL threshold correction unit, and a TFT threshold correction unit in the semiconductor device of the present invention. The figure which shows the Example of this invention. The figure which shows the Example of this invention. The figure which shows the Example of this invention.

Claims (2)

A pixel for driving the light emitting element;
The pixel includes first to third capacitor elements, first to sixth switch elements, a TFT, a capacitor line, a charge supply line, a signal line, and a charge voltage line.
The second electrode of the TFT is connected to the charge supply line, the first electrode is connected to the first electrode of the third switch element, and the gate is connected to the first electrode of the fifth switch element. connection,
A second electrode of the third switch element is connected to a first electrode of the first switch element;
The second electrode of the first switch element is connected to the first electrode of the first capacitor element and the first electrode of the second switch element;
A second electrode of the second switch element is connected to the second electrode of the fourth switch element and the light emitting element;
The first electrode of the fourth switch element is connected to the first electrode of the second capacitor element and the second electrode of the fifth switch element;
A first electrode of the third capacitor is connected to the control electrode of the second electrode and the third switch element of the sixth switching element,
A first electrode of the sixth switch element is connected to the signal line;
The second electrodes of the first and third capacitors are connected to the capacitor lines;
A second electrode of the second capacitor element is connected to the charging voltage line ;
In the first state, the sixth switch element is turned ON, and the third switch element is turned ON or OFF according to the voltage applied to the signal line,
In the second state, the first switch element is turned on, the second switch element is turned off, the fourth switch element is turned off, the fifth switch element is turned on, and the charging voltage line is turned on. On the other hand, a first voltage under the condition that the TFT is turned on is applied,
In the third state, the first switch element is turned off, the second switch element is turned on,
In the fourth state, the fourth switch element is turned on, the fifth switch element is turned off, and the second voltage is set to the same potential as the cathode voltage of the light emitting element with respect to the charging voltage line. A semiconductor device which is applied . A pixel for driving the light emitting element;
The pixel includes first to fourth capacitor elements, first to tenth switch elements, a TFT, a capacitor line, a charge supply line, a signal line, and a charge voltage line.
First electrode of the tenth switching element is connected to the charge supply line, a second electrode connected to a first electrode of the second electrode and the eighth switch element of the TFT,
The first electrode of the TFT is connected to the first electrode of the first switch element, the second electrode of the seventh switch element, and the first electrode of the ninth switch element; A gate connected to the second electrode of the eighth switch element and the first electrode of the fourth capacitor element;
The second electrode of the first switch element is connected to the first electrode of the first capacitor element and the first electrode of the second switch element;
The second electrode of the second switch element is connected to the first electrode of the third switch element and the second electrode of the fourth switch element;
A second electrode of the third switch element is connected to the light emitting element, and a control electrode is connected to the first electrode of the third capacitor element and the second electrode of the sixth switch element;
The first electrode of the fourth switch element is connected to the first electrode of the second capacitor element and the second electrode of the fifth switch element;
The first electrode of the fifth switch element is connected to the first electrode of the seventh switch element and the second electrode of the fourth capacitor element;
A first electrode of the sixth switch element is connected to the signal line;
The second electrode of the first and third capacitor elements and the second electrode of the ninth switch element are connected to the capacitor line,
A second electrode of the second capacitor element is connected to the charging voltage line ;
In the first state, the sixth switch element is turned ON, and the third switch element is turned ON or OFF according to the voltage applied to the signal line,
In the second state, the first switch element is turned on, the second switch element is turned off, the fourth switch element is turned off, the fifth switch element is turned on, and the charging voltage line is turned on. On the other hand, the first voltage under the condition that the TFT is turned on is applied, the seventh to ninth switch elements are turned off, the tenth switch element is turned on,
In the third state, the first switch element is turned off, the second switch element is turned on,
In the fourth state, the fourth switch element is turned on, the fifth switch element is turned off, and the second voltage is set to the same potential as the cathode voltage of the light emitting element with respect to the charging voltage line. Applied,
In the fifth state, the first switch element is turned off, the fifth switch element is turned off, the seventh to ninth switch elements are turned on, the tenth switch element is turned off, and the capacitance 3. A semiconductor device, wherein a third voltage of the line is lower than a fourth voltage of the charge supply line .

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